Add convenience function "clean"

[odysseus9672]

Add a convenience function named clean that calls rm then add on its argument in sequence. The justification is that it's a way to "clean" a local package that has been marked as "dirty", and return to the clean master version. This function is based on the recommendation of @greg_plowman in response to a question on how to revert dirty packages.
Suggested future improvements: only clean when the package is actually marked as dirty, use git reset --hard type functionality, as suggested by @stevengj.

[tkelman]

This actually doesn't do what you want. Pkg.rm is somewhat poorly named and doesn't do what you think, it doesn't delete any files. It moves the package to a .trash folder, leaving any modifications in place, in case something else gets added that depends on it or the package gets re-added. So all this operation will do is move the package to .trash then move it back, not fixing whatever local modifications you had. See #7054

[ararslan]

It should be pretty easy with the LibGit2 bindings to determine whether the package repository is in a dirty state and if so do a hard reset, otherwise do nothing.