Add optional post_install method to Formula.

Sometimes we may want to run commands after bottle installation (such as
creating directories outside the Cellar) so this method allows us to do
so.

Closes Homebrew/homebrew#18382.

Library/Homebrew/formula.rb

@@ -188,6 +188,9 @@ class Formula
   # are supported.
   def pour_bottle?; true end
 
+  # Can be overridden to run commands on both source and bottle installation.
+  def post_install; end
+
   # tell the user about any caveats regarding this package, return a string
   def caveats; nil end
 

Library/Homebrew/formula_installer.rb

@@ -111,6 +111,8 @@ class FormulaInstaller
       clean
     end
 
+    f.post_install
+
     opoo "Nothing was installed to #{f.prefix}" unless f.installed?
   end