Add martini templates (.tmpl) to mimetypes

This adds the extension .tmpl, which is an extension used by the Martini web framework, for Go html templates.

Added this to text/html section of mimetypes, not sure if this is the "right" section to add this to.

martini-contrib-render uses the two extensions .tmpl and .html by default for Go templates (although this is configurable), these are just plain Go html templates:

https://github.com/codegangsta/martini-contrib/tree/master/render

Note: I am not sure if this should be added to a different mimetype other than text/html, for example text/x-go-template.