Don't use keg-only and pouring jargon.

This is unnecessarily confusing to end-users and the "fun" naming doesn't really communicate anything. @mistydemeo and I talked about this.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
I would suggest not replacing "kegs" with "packages".  Better to just leave it as "kegs" (or change it to "brews").

Even if the adjective "keg-only" is confusing, surely the noun "keg" is not.  It's no more confusing (or jargony) than "Python/egg", "Ruby/gem", "Lua/rock" and so forth.  More importantly, "keg" is not any more confusing than "package".  They're just the things that Homebrew installs.