add support for supplying path extensions

When serving static files, it's often useful to be able to not include file names in the URIs.
For example, a request to `/hello` should try to serve `/hello.html` if file `hello` doesn’t exist.

I haven't been able to find a different simple way to do it in express, so I looked at the code and it seems `send` is the best place to do it. Therefore, this pull request implements the functionality in `send`. 

This would be my first pull request, so please be kind and advise if I'm doing something wrong or I'm missing something that should be obvious.