Reduce duplicate code between ado2gh command classes/test and gei command classes/tests

[dylan-smith]

Not sure what the solution is here, but it's obvious we've got a bunch of duplicated code/commands between the 2 CLI's, and for each duplicated command we've duplicated all the unit tests.

The challenge is that even ones that appear identical may have slightly different argument names.

One thought is to create a common Command class that lives in the Octoshift assembly, then have a way to override/tweak things in a small CLI specific file (maybe inheriting from the common Command class).