Add rdmd and rdmu aliases to Rails plugin

* `rdmd` is aliased to `rake db:migrate:down`
* `rdmu` is aliased to `rake db:migrate:up`
* These tasks allow you to run a specific migration up or down by
specifying the appropriate `VERSION` env variable, which contains
the numerical prefix of the migration's filename

plugins/rails/rails.plugin.zsh

@@ -49,7 +49,9 @@ alias rsp='rails server --port'
 
 # Rake aliases
 alias rdm='rake db:migrate'
+alias rdmd='rake db:migrate:down'
 alias rdms='rake db:migrate:status'
+alias rdmu='rake db:migrate:up'
 alias rdr='rake db:rollback'
 alias rdc='rake db:create'
 alias rds='rake db:seed'