Another method to define a private key

Added the ability to define a private key, placed in the default location (~/.ssh), just by its file name. (In case there are many keys to choose from.)
Does this work on Windows? Might want to use `path.sep` instead.

Also, this should probably do an `else if` so it doesn't always get set in the rare case that there happens to be a key in both locations, such as `./keyname.pub`.