invoke _compileTemplate with additional third arg

[kayomarz]

If the _compileTemplate() hook is invoked with an additional arg - filePath of the template, it means it knows the file extensions of that template. This helped me to use markdown along with express-handlebars engine in a clean way. This might be useful, in general for integrating with other markup/template technologies.

More explanation in the README of the master branch: https://github.com/kayomarz/express-handlebars.