Can't create index on TextField. Database is MySQL
Yeah, that's a limitation of MySQL -- you need to specify a length to index when you have a `TextField`.

When I run create_table() on a model with a TextField(index=True) column, it fails with this error:

```
peewee.OperationalError: (1170, "BLOB/TEXT column 'keywords' used in key specification without a key length")
```

The table is created successfully, but the index is not.
Here's a simple example which reproduces the error using my setup:

``` python
import peewee
from playhouse.pool import PooledMySQLDatabase

class TestModel(peewee.Model):
    keywords = p.TextField(index=True)

    class Meta:
        database = PooledMySQLDatabase("testdb", user="user", passwd="password")

TestModel.create_table()
```

```
peewee==2.7.4
MySQL-python==1.2.5
mysql  Ver 14.14 Distrib 5.7.9, for osx10.10 (x86_64) using  EditLine wrapper
```
