Create index with operator classes
For now the easiest way may be:

```python
class Taxonomy(Model):
    name = CharField()
    # ...

    class Meta:
        indexes = [
            SQL('create index on taxonomies_names (lower(name) varchar_pattern_ops...'),
        ]
```

Alternatively, you can:

```python

from peewee import NodeList

name = NodeList((fn.LOWER(Taxonomy.name), SQL('varchar_pattern_ops')))
index = (Taxonomy
         .index(name, Taxonomy.name_class)
         .where(Taxonomy.name_class == 'scientific name'))
Taxonomy.add_index(index)
```

The above will result in:

```python
CREATE INDEX "taxonomy_name_class" ON "taxonomy" (
    LOWER("name") varchar_pattern_ops, "name_class"
) WHERE ("name_class" = 'scientific name');
```

Docs:

* http://docs.peewee-orm.com/en/latest/peewee/api.html#Model.index
* http://docs.peewee-orm.com/en/latest/peewee/api.html#Model.add_index
* http://docs.peewee-orm.com/en/latest/peewee/api.html#ModelIndex
Hi there!
Is it possible to create an index like this:
```sql
create index on taxonomies_names (lower(name) varchar_pattern_ops, name_class)
where name_class = 'scientific name';
```

Thanks!