Couldn't make this atomic, apologies.
Fixes a few things, like deps failing to build not aborting the install.
--force now works properly again.
Overall more robust code. I went back over it all and gave it a lot of
thought.
Cleaner separation of logic. Less code in brew, now the only code there is
ARGV handling, and basic sanity checks.
Not extending ARGV or ENV in global now as that would propagate to other tools
or utilities you may write.
