Support ECMAScript Stage-3 Proposals and Below

[brentlintner]

Currently we use Esprima, which only supports Stage-4 specs with a potential for dev releases.

Babylon supports (through plugins), along with babel-generator, the ability to parse stage-3 and below.

Current thoughts based on prototyping:

1. Support Esprima by default, mention support for Stage-3 and below in the project README (or similar, like you mentioned)
2. Optionally have babel support with a --babel flag or --esnext flag, and even possibly use presets the user has installed themselves, thereby keeping synt's prod deps lean

Note: This was created from #91.

[brentlintner]

Will be fixed with #101