Update/Refresh Field List
This would be a great feature but if you take a look at some of the existing schema migration tools you might be surprised how tricky this is.

Currently peewee has a helper in `playhouse.migrate` but i've only tested it with postgres. Schema migrations in sqlite are tricky because it only supports adding columns. For now these are the best options.

Hi!

I would like to suggest implementing some sort of refresh_table method, which will add/delete Fields of the Model. 

For example I have a model:

```
class Pets(Model):
  name = CharField()
```

Then I do `Pets.create_table()` now I have the relevant table in my database, after some time (usually during development) I decide that I need another field:

```
class Pets(Model):
  name = CharField()
  ownerName = CharField()
```

But now the Model isn't in sync with the table in the database, and I get Errors like:
`"Unknown column 't1.ownerName' in 'field list'"`

It would be quite useful have some sort of refresh/update/alter_table method that would update the relevant fields, instead of adding the fields manually or dropping and recreating the table.

Thoughts?
