Allow support for multiple directory index files

This allows me to call `htmlproofer` with multiple directory index files, as a comma-separated list. For example: ``` htmlproofer --directory-index-files=index.html,index.php ``` This is necessary for my own web site, where I have both `index.html` and `index.php` files. I couldn’t get the tests to run without #859 and #858, so those PRs would be dependencies.