PROTOTYPE: port to firefox devtools addon

This pull request is not intended to be merged as it is, I’m opening it to start evaluating if there's interest to integrate it into the project and, in that case to work on it until it will be ready to be merged (and then fix #59).


DONE:
- build firefox and chrome extension from the same sourcecode

TODO: 
- firefox extension seems to work correctly, but there’re some CSS issues to work out.
- integrate xpi build