Forcing reinstall in package.el

[Phil Hagelberg]

While doing development of packages using package.el, I often find
myself reinstalling packages using M-x package-install-from-buffer.
However, this is a bit cumbersome because I have to remember to delete
the old version of the package out of my .emacs.d first. It would be
nice for package-install-from-buffer to be able to take a prefix
argument or something that would delete the package before attempting
to install it.

I volunteer to implement this if it's desired.

-Phil