Make newt 'rc-aware'

[sjanc]

This would allow newt to look for rc tags (with proper info/warning message) when specified release tag is not in repo.

This would make release process simpler and less confusing when multiple release candidates are happening (currently we need to move tags while doing so which is confusing for users who downloaded repo in between of release candidates...)

[ccollins476ad]

Fixed by #313.