Add and use plist helper methods

These will be used to unify the label namespaces in embedded plists,
i.e. 'homebrew.mxcl.<formula>'.

plist_path returns the full path to a plist file located at the top
level of the keg; plist_path.basename can be used if just the filename
is needed.

c.f. Homebrew/homebrew#9346.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula.rb

@@ -208,6 +208,10 @@ class Formula
   # generally we don't want var stuff inside the keg
   def var; HOMEBREW_PREFIX+'var' end
 
+  # plist name, i.e. the name of the launchd service
+  def plist_name; 'homebrew.mxcl.'+name end
+  def plist_path; prefix+(plist_name+'.plist') end
+
   # Use the @spec_to_use to detect the download strategy.
   # Can be overriden to force a custom download strategy
   def download_strategy