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.

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.