Add auto-update functionality

[franok]

Just a thought, not sure if it's a good or bad idea.

Try to auto-update patch levels (or even minor versions?). Send notification about update availability, execute update, send notification about success/failure.

Pros:

• less manual interaction if updates are available
• registering hooks to call update-scripts, located next to the installed software

Cons:

• Cannot rely on SemVer. Software could also introduce breaking changes in patch-levels.
• Other software doesn't follow Semver.
• Danger of breaking something is high.