Improvements

[ciis0]

Hello,

I used your project to setup a GitHub (Enterprise) mirror that is kept in sync using webhooks and would like to contribute back the adjustments I made. :)

• I changed the behaviour of which branch to listen to (defaults now to all branches, more useful for mirroring)
• Changed processing of stdout/stderr of script
• Enabled handling for ping event (used for initial mirroring)
• added mirror script and systemd service

I have just commited this on the go, so this PR mixes this things up. If you prefer the changes in seperate PRs/branches I can look into that. :)

I think the ping handling and listening to all branches should maybe still be configurable, but I didn't look into that yet.

Thanks you. :)