`nanoc clean` command

[denisdefreyne]

Original ticket:

Akin to make clean, it would be nice if nanoc knew how to clean up after itself, hence the proposed nanoc clean. While it's true that one can employ ad-hoc cleanup methods (rm -fr crash.log output tmp, git clean -dXf, etc.) an officially supported method would be easier to document and more amenable to scripting.

Aside from the obvious build products (output and tmp), one would expect other nanoc-specific detritus (crash.log, output.diff, and any others) to be removed as well.