Support specifying a table comment

[booqoffsky]

I would like to be able to specify a comment on the table in the model, which will get into the database through migrations.

For example, it might look like this:

class MyModel(ormar.Model):

    class Meta:
        comment = "My Super Comment"
        

The 'comment' can be used in the populate_meta_sqlalchemy_table_if_required function as the 'comment' keyword argument when calling sqlalchemy.Table