playhouse.reflection.generate_models does not reflect index_type
Currently we don't support detection of special index-types.
Using peewee 3.13.1 with PostgreSQL 9.6.15 and psycopg2 2.8.2, I noticed that a model class generated with generate_models() had a column that did not reflect its specified `index_type`:

```python
# db is a FlaskDB object
class TestModel(db.Model):
    a_field = TextField(index=True, index_type="BRIN")

TestModel.create_table()
models = generate_models(db.database)
models["testmodel"].a_field.index
#> True
models["testmodel"].a_field.index_type
#> None 
```

Here is the table definition:

```
# \d testmodel
                             Table "public.testmodel"
 Column  |  Type   | Collation | Nullable |                Default
---------+---------+-----------+----------+---------------------------------------
 id      | integer |           | not null | nextval('testmodel_id_seq'::regclass)
 a_field | text    |           | not null |
Indexes:
    "testmodel_pkey" PRIMARY KEY, btree (id)
    "testmodel_a_field" brin (a_field)
```