cli: Add support for running custom commands or shell on the mock environment

[3v1n0]

It may be used to quickly debug interfaces

Allowing to use something like python3 -m dbusmock -t upower -c d-feet

[3v1n0]

Mh, not sure what's not making pyflakes to work here, as I get the test passing locally... mhmhm

However, this was mostly a RFC, so let me know if interested.

[3v1n0]

@martinpitt hey, a part the required fixes, do we have any interest on this?

I ideally would like to have a full interface to do something like dbus-run-session -- python3 -m dbusmock -t foo -t bar -- my-script --args

[3v1n0]

Rebased to fully use ctypes instead of gi.repository