revid-cli: Command-line flag to rotate video

[Alan Noble]

Raspivid supports image rotation, but revid-cli does not expose that functionality.

A simple "Rotate 180" command would achieve the same thing as a HorizontalFlip plus a VerticalFlip.

Rotation of smaller angles would also enable us to adjust for cameras that are installed slightly out of alignment w.r.t. the horizon (or seafloor).

[Saxon Milton]

Resolved in PR #158