Added -R option to include directories recursively.

Hi,

I was using this library for one of my projects and felt that it missed the option to concatenate *.coffee files found in subdirectories, so I coded one.

Now you can use the -R option to include a folder recursively. 

I basically just used the fs.readdir function instead of its synchronous counterpart, the latter only lists out files, but not subfolders inside a given folder.

the -I option still includes files non-recursively.