"Sync" gist: extend "update" to also remove files no longer present

[aseemk]

It'd be great to be able to sync a gist to a folder or list of files. --update can do most of this (add new files + update existing files); the only thing missing is removing files no longer present or specified.

A simple way to achieve this would to list the gist's files first (that functionality would be helpful on its own; issue #287) and match by filename. The Gist API then supports specifying a null file to delete it:

https://developer.github.com/v3/gists/#edit-a-gist

Thanks for the consideration!