Add allowSeeksWithinUnsafeLiveWindow property

[gesinger]

This property will prevent the playback watcher from attempting to
correct seeks that fall outside of the safe live window, but still fall
within the playlist boundaries.

It can help resolve issues where the playback watcher's gap skipper tries
to seek past a gap, but the playback watcher then detects an illegal seek
and corrects it to the safe live point, which, being content already
played, leads to repeated content.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors