Add "brew --cellar (formula)"

If a formula name is passed to "brew --cellar", output the location of that
brew in the Cellar (which will be "brew --prefix (formula)" without a version
sub-folder.)

This allows for:
  rm -rf `brew --cellar node`
for deleting all versions of node, installed or not.

bin/brew

@@ -88,7 +88,11 @@ begin
   when '--repository'
     puts HOMEBREW_REPOSITORY
   when '--cellar'
-    puts HOMEBREW_CELLAR
+    if ARGV.named.empty?
+      puts HOMEBREW_CELLAR
+    else
+      puts *ARGV.formulae.collect {|f| HOMEBREW_CELLAR+f.name}
+    end
   when '--config'
     dump_config