Add g4 parser
Currently we only have a few grammars enabled in this repo.
https://gitlab.com/coala/bears/coala-antlr/blob/master/.ci/downloadGrammars.sh
The complete set is at https://github.com/antlr/grammars-v4/
Part of the problem is that it is hard to know which .g4 antlr files are able to be used in Python.
We should the g4 grammar here, so we can parse the .g4 files to determine if they are suitable to be used as a grammar for this project.