Allow requirements to specify a default formula.

This allows default resolution of requirements without user intervention.

Closes Homebrew/homebrew#19627.

Formula/elixir.rb

@@ -2,6 +2,7 @@ require 'formula'
 
 class ErlangInstalled < Requirement
   fatal true
+  default_formula 'erlang'
 
   satisfy { which 'erl' }
 

Formula/mu.rb

@@ -3,6 +3,7 @@ require 'formula'
 class Emacs23Installed < Requirement
   fatal true
   env :userpaths
+  default_formula 'emacs'
 
   satisfy do
     `emacs --version 2>/dev/null` =~ /^GNU Emacs (\d{2})/

Formula/mysql-connector-odbc.rb

@@ -2,6 +2,7 @@ require 'formula'
 
 class MySqlInstalled < Requirement
   fatal true
+  default_formula 'mysql'
 
   satisfy { which 'mysql_config' }
 

Formula/shocco.rb

@@ -10,6 +10,7 @@ end
 
 class MarkdownProvider < Requirement
   fatal true
+  default_formula 'markdown'
 
   satisfy { which 'markdown' }
 

Formula/signing-party.rb

@@ -2,6 +2,7 @@ require 'formula'
 
 class GnupgInstalled < Requirement
   fatal true
+  default_formula 'gnupg2'
 
   satisfy { which('gpg') || which('gpg2') }