Added rmdir function for compatable with fs.rmdir

[meefik]

Added rmdir function as clone of unlink function. Function fs.unlink unsupported delete of directory, this fix for better compatibility with fs module.

[perry-mitchell]

Thanks @meefik. This is definitely a wise addition. I think in the next major version might have to validate targets before deletion if both unlink and rmdir are to be used in different ways (files vs directories). A true fs module would differentiate between files and directories, whereas this library currently does not. Need to consider whether or not it's worthwhile making it 2 requests instead of 1.

[perry-mitchell]

Thanks @meefik