Add optional rename function for custom module identifier name

We wanted to only use the templateCache in our production environment so we wanted our templateUrl to be compatible with both the module identifier created with grunt-html2js as well as our Express web server serving up .ejs files (which are basically static html files for our angular templates) in our dev environment.

Using a rename function we can add a leading slash and remove the .ejs from the module identifier keeping it the same as what we use in our dev environment. 

Our existing templateUrl: `/directives/user-badge`
Output before the rename function: `directives/user-badge.ejs`
Output after the rename function: `/directives/user-badge`

Please let me know if there is a better way to do something like this in the existing grunt-html2js. Thanks!