js parser fixes: jsx and backquoted strings

Summary:
Fixed JS parser to implement the following:
1. JSX tags can now have colons.
2. ES6 adds the ability to insert arbitrary strings, including quotes
without escape, between a pair of backquote symbols.

Test Plan:
./pfff -dump_js tests/js/parsing/backquote.js
./pfff -dump_js	tests/js/parsing/xhp_colon.js

Reviewers: pad

Reviewed By: pad

CC: bhosmer, jeffmo, bryano

Differential Revision: https://phabricator.fb.com/D1198444