Support scanning GitHub groups and repositories

[scastlara]

Is your feature request related to a problem? Please describe.
We should be able to:

• Scan GitHub groups
• Scan GitHub repos
• Publish full reports as GitHub issues.

Describe the solution you'd like
Feature parity between Gitlab and GitHub (am I missing something above?).

Describe alternatives you've considered
...
Additional context
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[scastlara]

Before jumping into coding for this, maybe we could sketch how we would like the cli opts to look like? Since now it is pretty tied to gitlab.

Should we have separate options for GitHub and gitlab? If so, what if they are passed together?

Should we auto detect the platform of the given repo or group (org in GitHub)? What about self hosted gitlab instances? Would they be easy to auto detect? If we do this, we need to rename the cli opts to be somewhat generic.

[scastlara]

Checkout #17 for a cli proposal