Add brew --cellar command.

The Cellar may not actually be "`brew --prefix`/Cellar".
We support the Cellar existing only in the repo but
not linked into the prefix, for installs that aren't
directly in /usr/local (or other chosen prefix.)

bin/brew

@@ -77,6 +77,8 @@ begin
     puts HOMEBREW_PREFIX
   when '--repository'
     puts HOMEBREW_REPOSITORY
+  when '--cellar'
+    puts HOMEBREW_CELLAR
   when '--config'
     dump_config