Guess if .r is R or REBOL

.r is the default extension used widely for both, R and REBOL. This
patch attempts to guess if .r is a REBOL file by looking for the
following very common REBOL code fragments:

- `REBOL`
- `: func [`
- `make object! [`
- `context [`

If any of those is found in a .r file, it is identified as a REBOL file.
Otherwise the language for a .r file is R.

Signed-off-by: Andreas Bolka <a@bolka.at>

test/test_blob.rb

@@ -224,6 +224,10 @@ class TestBlob < Test::Unit::TestCase
     assert_equal Language['Ruby'],        blob("wrong_shebang.rb").language
     assert_nil blob("octocat.png").language
 
+    # .r disambiguation
+    assert_equal Language['R'],           blob("hello-r.R").language
+    assert_equal Language['Rebol'],       blob("hello-rebol.r").language
+
     # ML
     assert_equal Language['OCaml'],       blob("Foo.ml").language
     assert_equal Language['Standard ML'], blob("Foo.sig").language