geoalchemy2 support

## Help! geoalchemy2 support!

```

class CoordinateMixin():
location = db.Column('location', Geography(geometry_type='POINT' ,srid=4326, spatial_index=True, dimension=2), doc='gps coordinate')

```
Exception: Don't know how to convert the SQLAlchemy field user.location ()

I'v already add the following code in schema file.
```

from geoalchemy2 import Geography
from graphene_sqlalchemy.converter import get_column_doc, convert_sqlalchemy_type
from app.graphql.extesions import CoordinateJSON

@convert_sqlalchemy_type.register(Geography)
def convert_column_to_coordinatejson(type, column, registry=None):
return graphene.Field(CoordinateJSON, description=get_column_doc(column))

schema = graphene.Schema(query=RootQuery, types=types, mutation=MyMutation)

```

先從 schema 檔案和回報的轉換例外開始，接著檢查 graphene_sqlalchemy.converter 如何分派 SQLAlchemy 欄位類型。重現 Geography 範例，並驗證 schema 建立能處理 location 欄位並產生預期的 CoordinateJSON 欄位。

由索引模型根據 Issue 內容生成。