Allow snake_case module and class naming in generated proto files

[film42]

Resolves the following:

Offenses:

spec/support/protos/google_unittest.pb.rb:15:8: C: Use CamelCase for classes and modules.
module Protobuf_unittest
       ^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest.pb.rb:69:9: C: Use CamelCase for classes and modules.
  class OptionalGroup_extension < ::Protobuf::Message; end
        ^^^^^^^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest.pb.rb:70:9: C: Use CamelCase for classes and modules.
  class RepeatedGroup_extension < ::Protobuf::Message; end
        ^^^^^^^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest.pb.rb:367:9: C: Use CamelCase for classes and modules.
  class OptionalGroup_extension
        ^^^^^^^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest.pb.rb:371:9: C: Use CamelCase for classes and modules.
  class RepeatedGroup_extension
        ^^^^^^^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest_import.pb.rb:14:8: C: Use CamelCase for classes and modules.
module Protobuf_unittest_import
       ^^^^^^^^^^^^^^^^^^^^^^^^
spec/support/protos/google_unittest_import_public.pb.rb:8:8: C: Use CamelCase for classes and modules.
module Protobuf_unittest_import
       ^^^^^^^^^^^^^^^^^^^^^^^^

cc @abrandoned @mmmries @liveh2o @brianstien

[nerdrew]

Same as #287. Want me to close mine?

[film42]

Whoops! Sorry about that @nerdrew . I'll close mine!

[film42]

@nerdrew I merged yours. Thanks for your help!