Migration to change CharField max_length
Indeed, for that you will need to probably consult the postgres manual for the correct SQL.  Peewee handles most things, but this is one of the ones it doesn't.

The title says the most of it. I'm using `arnold` for migrations on my PostgreSQL database, and I think I'd want to do something like: 

```
def up():
    config.DB.execute_sql("SOME arbitrary SQL to MODIFY column;")

def down():
    config.DB.execute_sql("OPPOSITE of ABOVE SQL code;")
```

Does that sound about right? I don't see any first-class features for changing the size of a Postgres VARCHAR.
