Add a 'rename' option

Added in a rename option allowing fine grained control on the file url (and a test too).
Documented the rename option in `README.md`.

For a bit of context, I have a project where each template is placed inside a `/templates` folder. This resulted in template urls like, `module/templates/list.html`. The rename option let's me strip out `/templates` really easily. 

Let me know your thoughts/if there are any issues with the code!