Adding an 'add migration' script to create a groovy migration file placeholder

...and automatically add it to the changelog file.

Have missed something like this for a while and thought I might as well try to get the ball rolling on adding something like this to the core plugin.  Groovy only right now - not sure how to make it XML as well.

Definitely open to learning how to do this a better way, but as I show Grails to more people, I realize manual migration creation is an extra step.  Having this be part of the core migrations would make it easier for people to get in to the migration habit.