Help with alter table queries
Decided that the range of possible alter table queries really precludes a general purpose solution that wouldn't involve a whole lot of code.  Going to leave this up to the end-user to handle if it goes beyond the most basic case (add/drop/rename col).

Although there are no automated 'migrations' in peewee, it would be helpful to give users a way to get column sql and generate alter table queries (or with sqlite, since there's no drop column, maybe the temptable thing).
