Add Starlark support to Python lexer (#1369)

Starlark is a dialect of Python that is used to configure the Bazel
build system. This adds file extensions to the Python lexer that are
used for files written in Starlark.

lib/rouge/lexers/python.rb

@@ -8,7 +8,8 @@ class Python < RegexLexer
       desc "The Python programming language (python.org)"
       tag 'python'
       aliases 'py'
-      filenames '*.py', '*.pyw', '*.sc', 'SConstruct', 'SConscript', '*.tac'
+      filenames '*.py', '*.pyw', '*.sc', 'SConstruct', 'SConscript', '*.tac',
+                '*.bzl', 'BUCK', 'BUILD', 'BUILD.bazel', 'WORKSPACE'
       mimetypes 'text/x-python', 'application/x-python'
 
       def self.detect?(text)

spec/lexers/python_spec.rb

@@ -14,6 +14,11 @@
       assert_guess :filename => 'SConstruct'
       assert_guess :filename => 'SConscript'
       assert_guess :filename => 'foo.tac'
+      assert_guess :filename => 'foo.bzl'
+      assert_guess :filename => 'BUCK'
+      assert_guess :filename => 'BUILD'
+      assert_guess :filename => 'BUILD.bazel'
+      assert_guess :filename => 'WORKSPACE'
     end
 
     it 'guesses by mimetype' do