Add JSHint tool plugin.

This adds support for running JSHint on JavaScript code. There are 3 options
that allow an admin to specify whether message codes should be included, which
file extensions JSHint should run on (besides .js), and whether JSHint should
extract JavaScript from HTML files.

Testing Done:
Saw the JSHint tool show up in the "Installed Tools" list after refreshing it.

Created, modified, and deleted the tool.

Published review requests with .js and .html files, using various combinations
of options for the tool:
- default options (verbose=False, extra_ext_checks='',
  extract_js_from_html='never')
- verbose=True, extra_ext_checks='', extract_js_from_html='never'
- verbose=True, extra_ext_checks='.html,.json', extract_js_from_html='auto'
- verbose=True, extra_ext_checks='.html', extract_js_from_html='always'
- verbose=True, extra_ext_checks='.html,.json', extract_js_from_html='never'

In all cases, I saw the expected results.

Reviewed at https://reviews.reviewboard.org/r/6234/