[purge] Add --include and --exclude and a 'clean' alias

[Emanuele Aina]

I've added the usual --include and --exclude options to the purge extension.

In a separate commit I've added the 'clean' alias for purge to make the 
transition easier for other SCM refugees, as svn has svn-clean and git 
git-clean.

Both changes are in my repository at http://techn.ocracy.org/hg-em/ but 
I can send the patches here if you prefer.

-- 
Buongiorno.
Complimenti per l'ottima scelta.