Make jshint binary configurable

Adds an option to use a custom binary. I'm using this for running `jsxhint`, since `SublimeLinter-jsxhint` just doesn't seem to work correctly, and I don't like using the `ignore` directive.

Issues with this PR:
1) This only makes the binary used in JSHint Text Command configurable. Not quite sure how to update the build command, but it's probably possible.
2) Didn't specify a specific User Preferences file for this package.

If this feature will be useful for anyone, I can address those two issues.