CompileDir Implementation

Including a CompileDir function makes it a lot easier for users to cache their templates to be accessed when needed in handlers. Detailed documentation in README.

Added this function to compiler.go, with a new DirOptions struct to specify what extensions to match and whether or not the search should be recursive.

Added tests to amber_test.go, and files in samples/ for testing.

I'm quite new to Go, so if there's anything wrong just let me know and I'll fix it - or if you think this functionality is beyond the scope of this project that's fair too.

Thanks!