Support creating a table migration generator

Sometimes you want to create a table without an associated model and
test, which is also not a join table. With this commit, you can now
do that.

Example:

    rails g migration create_posts title:string
or
    rails g migration CreatePosts title:string

This commit also moves the template the model generator uses for the
migration to the migration templates folder, as it seems a more
sensible place for it now that it is shared code.