improve clean command

[pietroppeter]

in recent PR #56 clean command has been reduced to only clean the output folder.
it could still use some improvements:

• add a dry-run option
• how to cleanup automatically logs?
• support the keep parameter taking values from nimibook config (so that we can avoid cleaning specific files in output folder
• should we reintroduce an option to perform again cleaning of source folder? (we would need keep to support source folder also)

[HugoGranstrom]

how to cleanup automatically logs?

I don't know what it's worth, but logs are removed at the start of building a document. So if it runs successfully, the old log file will be removed (or rather, it will not be recreated).