Add '--owner' required arg for github endpoint

Github doesn't require that projects be uniquely named.  Instead, it
uses <owner-name,repository-name> tuples to uniquely identify a
repository.

This means that bugging-out also needs to know the owner of a
repository before it can fetch issues using Github's API.  This commit
adds the '--owner' argument, which is a required argument only if the
endpoint type is 'github'.

This change in the code pushes the argument-handling code nearer a
tipping point.  Initially, the code was simple enough to be written
in a non-OOP fashion, as most of the work was done by getopt.

However, as the logic has evolved to support required arguments,
arguments with only certain acceptable values, and now conditionally
required arguments, the code is starting to become unwieldy.

For now, the unit tests have kept things working, but at the end of
this sprint, the code should be refactored to be written in a more
OOP friendly fashion.

Issue #7