Override languages being included by language statistics (#3807)

* Add detectable key to languages

This key allows to override the language being included in the
language stats of a repository.

* Make detectable override-able using .gitattributes

* Mention `linguist-detectable` in README

* Remove detectable key from languages

Reverts changes in 0f7c0df5.

* Update commit hash to the one that was merged

PR #3806 changed the commit hash. The original commit was not
actually merged into the test/attributes branch.

* Fix check to ensure detectable is defined

* Add include in language stats tests when detectable set

* Ignore detectable when vendored, documentation or overridden

* Add documentation on detectable override in README

* Improve documentation on detectable override in README

lib/linguist/blob_helper.rb

@@ -383,7 +383,10 @@ module Linguist
       !vendored? &&
       !documentation? &&
       !generated? &&
-      language && DETECTABLE_TYPES.include?(language.type)
+      language && ( defined?(detectable?) && !detectable?.nil? ?
+        detectable? :
+        DETECTABLE_TYPES.include?(language.type)
+      )
     end
   end
 end

lib/linguist/lazy_blob.rb

@@ -7,7 +7,8 @@ module Linguist
     GIT_ATTR = ['linguist-documentation',
                 'linguist-language',
                 'linguist-vendored',
-                'linguist-generated']
+                'linguist-generated',
+                'linguist-detectable']
 
     GIT_ATTR_OPTS = { :priority => [:index], :skip_system => true }
     GIT_ATTR_FLAGS = Rugged::Repository::Attributes.parse_opts(GIT_ATTR_OPTS)
@@ -70,6 +71,14 @@ module Linguist
       end
     end
 
+    def detectable?
+      if attr = git_attributes['linguist-detectable']
+        return boolean_attribute(attr)
+      else
+        nil
+      end
+    end
+
     def data
       load_blob!
       @data