Add ESLint support to JS linting

Adds preliminary support for [ESLint](https://github.com/nzakas/eslint) to the JavaScript linting config. As there is not currently a single-file build of ESLint, the entire source is pulled in via a submodule. This will change as soon as a stable single-file build of ESLint is available.

Because of this, configuring SublimeLinter to use ESLint is currently incredibly slow, as ESLint has to read a large number of files (via the Node `fs` module) every time something changes in a source file.

*This PR is effectively a proof-of-concept. I'm not expecting it to be merged in its current state*.