Enabling custom template extension

Hi guys, when working out with express.js and its connect handlebars module, I find extremely useful to work with `hbs` extension because it keeps everything shorter and is the default one.
However, when adding custom build scripts to precompile templates I find it annoying as the `hbs` extension remains in the template name.

With this commit I added an additional, optional, parameter to specify the template extension so that it could get easily removed from template name. The parameter has its default value set to `handlebars` as to be consistent with the current implementation.

Thank you for your great work!