rails3, added rgm = 'rails generate migration'

plugins/rails3/rails3.plugin.zsh

@@ -13,6 +13,7 @@ alias rd='_rails_command destroy'
 alias rdb='_rails_command dbconsole'
 alias rdbm='rake db:migrate db:test:clone'
 alias rg='_rails_command generate'
+alias rgm='_rails_command generate migration'
 alias rp='_rails_command plugin'
 alias ru='_rails_command runner'
 alias rs='_rails_command server'