JavaScript support for Infer

This issue is created at the suggestion of @jvillard ([see conversation](https://gitter.im/facebook/infer?at=5e203494dd1429262d349088)) to get the ball rolling for possible JavaScript support.

I have been playing around with things off and on for a few months. The general strategy I've been using is (1) flow parser to parse js files (per file basis), (2) frontend handles translating flow parser representation into SIL.

My work has been all local so far, but I have just created a new fork for the purposes of adding JS support: https://github.com/avery-laird/infer . I will be integrating my changes to this fork over time, and anyone who is interested in contributing is welcome and encouraged to do so.