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.
Ensure deleting a key deletes the comment
Fix interpolation bug
Merge pull request #1 from timblair/handle-comments

Handle deleting keys with added comments