Add .jshintrc

This adds a `.jshintrc` to the `linters/` directory. I just copied over
the settings from the `SublimeLinter` file, de-nesting the JSON object
by one level.

It means we'll have to maintain both files, but it's good to have a
basic `.jshintrc` for those of us that aren't using JSHint through
Sublime Text.

As requested in https://github.com/airbnb/rendr/pull/239.

@hshoff what do you think?