Url normalization

This module uses `normalize-url`, which by default strips (among other things) the `www` from the URLs. I had a use case where I did not want this to happen.

I have no idea whether if there is a way for `mongoose-type-url` to receive additional parameters which would be passed on to configure `normalize-url`. If this is not possible, at least referring `normalize-url` in this module's documentation would make it more clear.

Start by locating where the module calls normalize-url and review the module documentation. Determine whether normalization options can be passed through the URL field configuration; done means the requested www-preserving behavior is supported or the normalize-url dependency and its relevant options are clearly documented.

Written by the indexing model from the issue text.