Add stats as a github step output

[damccorm]

This will give users the ability to manipulate the outcome of a run themselves and connect to arbitrary output sources. I tried this myself and was able to send a reasonably well formatted email with minimal work (see https://github.com/damccorm/playground/blob/main/.github/workflows/pr-stats.yml for my example).

Fixes #72

[damccorm]

Update: I got this to a point where I feel pretty good about the email formatting, so this approach definitely can work :)

[damccorm]

Thanks!

[manuelmhtr]

Hi @damccorm thanks for your help! Changes are available in v2.11.0. Just made a couple of changes:

1. Renamed results to resultsMd (to specify this is the markdown output).
2. Added the variable resultsJson as the results in JSON format.

Best!