Add CLI option for local HTML_CodeSniffer

Added an optional CLI option to specify a URL to soure the HTML_CodeSniffer JavaScript from. Fixes #10.

To use a local copy of HTML_CodeSniffer, clone the repository and start a HTTP server in the root directory. For example,

```bash
git clone git://github.com/squizlabs/HTML_CodeSniffer.git
cd HTML_CodeSniffer
python -m SimpleHTTPServer # starts a server on port 8000
```

And run `pa11y -c 'http://localhost:8000/HTMLCS.js' foo.com`.