Add command line flag "--ext" (short "-e") to specify a list of file extensions to watch

[vladvelici]

Addresses issue https://github.com/codegangsta/gin/issues/44. Restarting on template change can be achieved using flags "-e .go -e .html".

I needed to restart the server when I edit templates, and I see others have this issue too. This is a really simple solution by just adding a command line flag --ext (short -e), which is an array of strings, so one can do, for example:

gin -e .go -e .html

I would really appreciate any feedback.

Thanks Vlad