Handle deleting keys with added comments

The format for an authorized key permits a comment on end of the key line (separated from the key itself with a space).  GitHub's API include anything in this field, but this change means the user can manually add comments and still expect usage: `gh-auth remove` to work.

Also potentially allows for automatically adding a `user@github`-style comment to each line added to `authorized_keys` to track [where the keys have come from](https://github.com/timblair/github-auth/compare/handle-comments...add-username-comment) (WIP).