Pass routes through if they're defined in _config.yml

This allows you to define a new key in __config.yml_ called `auth_whitelist`. They're defined as an array of strings, but these strings are interpreted as regexes to make it easier to consume.

If jekyll-auth detects the route matches a whitelist, no authentication is necessary. If no `auth_whitelist` is provided, the assumption is that every path should be blocked (current behavior).

/cc @benbalter