Optional Mustache Support

To provide some options - I feel users migrating into using hem as a general quick off the ground server/bundling solution would benefit from having optional mustache template support.

Also, with the way I have written the code, it makes the mustache usage optional. While I picked hogan (because I'm using it in my current project) it would be easy to add different engine support in the future or if someone else wanted to add different support.

If someone added the option for a different mustache template handler - then it would be the same pattern but the user should only have one mustache engine installed or the last one in the code would win.