-f flag to overwrite existing output file

[coreyog]

I'm using your project to include shader language files in my Go-GL project. I'm not serving the files over HTTP, I just like having the resources embedded into the binary so it's all in one single package. It works great but I find that I'm modifying my external shaders quite often as I develop my app. I have to delete statik/statik.go each time before I run statik again. I can see the value in not overwriting files on the disk but I think a -f force flag would make my life a lot easier.