[Feature] option for Generate/Watch that leaves the temp directory when an error occurs

[kekonn]

Right now I am left with an undebuggable issue that the GenerateCommand is complaining about something in the generated html, but the StaticGenerator is deleting the temp directory, the error then exits and the docs directory is deleted. This way I don't have anything to inspect.

An argument -nc or --no-cleanup would be nice to have, together with a Clean action that runs the cleanup seperately.