Add .bzl extension to Python in languages.yml

.bzl is the extension for Skylark, a subset of Python that is used
to define build extensions for the Bazel build system.

Eg:
  https://github.com/bazelbuild/bazel/blob/master/tools/build_rules/closure/closure_js_binary.bzl

samples/Python/closure_js_binary.bzl

@@ -0,0 +1,124 @@
+# Copyright 2015 The Bazel Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Build definitions for JavaScript binaries compiled with the Closure Compiler.
+
+A single file is produced with the _compiled.js suffix.
+
+By default, the name of the entry point is assumed to be the same as that of the
+build target. This behaviour may be overridden with the "main" attribute.
+
+The optimization level may be set with the "compilation_level" attribute.
+Supported values are: unobfuscated, simple, and advanced.
+
+Example:
+
+  closure_js_binary(
+      name = "hello",
+      compilation_level = "simple",
+      language_in = "ecmascript6",
+      language_out = "ecmascript3",
+      externs = ["//third_party/javascript/google_cast/cast.js"],
+      deps = [
+          "@closure_library//:closure_library",
+          ":hello_lib",
+      ],
+  )
+
+This rule will produce hello_combined.js.
+"""
+
+_COMPILATION_LEVELS = {
+  "whitespace_only": [
+      "--compilation_level=WHITESPACE_ONLY",
+      "--formatting=PRETTY_PRINT"
+  ],
+  "simple": ["--compilation_level=SIMPLE"],
+  "advanced": ["--compilation_level=ADVANCED"]
+}
+
+_SUPPORTED_LANGUAGES = {
+  "es3": ["ES3"],
+  "ecmascript3": ["ECMASCRIPT3"],
+  "es5": ["ES5"],
+  "ecmascript5": ["ECMASCRIPT5"],
+  "es5_strict": ["ES5_STRICT"],
+  "ecmascript5_strict": ["ECMASCRIPT5_STRICT"],
+  "es6": ["ES6"],
+  "ecmascript6": ["ECMASCRIPT6"],
+  "es6_strict": ["ES6_STRICT"],
+  "ecmascript6_strict": ["ECMASCRIPT6_STRICT"],
+  "es6_typed": ["ES6_TYPED"],
+  "ecmascript6_typed": ["ECMASCRIPT6_TYPED"],
+}
+
+def _impl(ctx):
+  externs = set(order="compile")
+  srcs = set(order="compile")
+  for dep in ctx.attr.deps:
+    externs += dep.transitive_js_externs
+    srcs += dep.transitive_js_srcs
+
+  args = [
+      "--entry_point=goog:%s" % ctx.attr.main,
+      "--js_output_file=%s" % ctx.outputs.out.path,
+      "--dependency_mode=LOOSE",
+      "--warning_level=VERBOSE",
+  ] + (["--js=%s" % src.path for src in srcs] +
+       ["--externs=%s" % extern.path for extern in externs])
+
+  # Set the compilation level.
+  if ctx.attr.compilation_level in _COMPILATION_LEVELS:
+    args += _COMPILATION_LEVELS[ctx.attr.compilation_level]
+  else:
+    fail("Invalid compilation_level '%s', expected one of %s" %
+         (ctx.attr.compilation_level, _COMPILATION_LEVELS.keys()))
+
+  # Set the language in.
+  if ctx.attr.language_in in _SUPPORTED_LANGUAGES:
+    args += "--language_in=" + _SUPPORTED_LANGUAGES[ctx.attr.language_in]
+  else:
+    fail("Invalid language_in '%s', expected one of %s" %
+         (ctx.attr.language_in, _SUPPORTED_LANGUAGES.keys()))
+
+  # Set the language out.
+  if ctx.attr.language_out in _SUPPORTED_LANGUAGES:
+    args += "--language_out=" + _SUPPORTED_LANGUAGES[ctx.attr.language_out]
+  else:
+    fail("Invalid language_out '%s', expected one of %s" %
+         (ctx.attr.language_out, _SUPPORTED_LANGUAGES.keys()))
+
+  ctx.action(
+      inputs=list(srcs) + list(externs),
+      outputs=[ctx.outputs.out],
+      arguments=args,
+      executable=ctx.executable._closure_compiler)
+
+  return struct(files=set([ctx.outputs.out]))
+
+closure_js_binary = rule(
+    implementation=_impl,
+    attrs={
+        "deps": attr.label_list(
+            allow_files=False,
+            providers=["transitive_js_externs", "transitive_js_srcs"]),
+        "main": attr.string(default="%{name}"),
+        "compilation_level": attr.string(default="advanced"),
+        "language_in": attr.string(default="ecmascript6"),
+        "language_out": attr.string(default="ecmascript3"),
+        "_closure_compiler": attr.label(
+            default=Label("//external:closure_compiler_"),
+            executable=True),
+    },
+    outputs={"out": "%{name}_combined.js"})