Allow the script to be used even if no default config file was found

This change allows for running the script entirely by terminal.

See: https://github.com/IQAndreas/github-issues-import/issues/20

This also uses the "friendly" error message if no config file was found
(there was already a friendly error message system in there, but
apparently Python throws a different error depending on the OS. This one
at least catches the error mentioned in the issue.)
Add option to ignore even the default config file
Fix config file use and options

Now the default config file is optional.

Also adds the ability to ignore the config file entirely with `--config none`.
Apply the `--source` and `--target` arguments if set

Doh.
Fix tiny whitespace issue

A whole commit just for this little thing?! Preposterous! Yet, I seem to
be left with no other choice, and my concience does not allow me to
sneak it in with an unrelated commit.

Ugh...