add master/slave automatic selection when readonly sql

I have two database engine . one is readonly.
use the one  when the sql is like "select ..."  or  "show ..."

```python
SQLALCHEMY_DATABASE_URI = 'mysql://%s:%s@%s:%s/%s' % (_DBUSER, _DBPASS,
_DBHOST, _DBPORT, _DBNAME)

SQLALCHEMY_BINDS = {
'slave':  'mysql://%s:%s@%s:%s/%s' % (_DBUSER, _DBPASS, _DBHOST_S,
_DBPORT, _DBNAME)
}
```

=========================================
This is not use in _bind_key  way.

may be we count defind somethink like this:
```python
SQLALCHEMY_DATABASE_URI = 'mysql://%s:%s@%s:%s/%s' % (_DBUSER, _DBPASS,
_DBHOST, _DBPORT, _DBNAME)
SQLALCHEMY_DATABASE_URI_SLAVE = 'mysql://%s:%s@%s:%s/%s' % (_DBUSER, _DBPASS, _DBHOST_S,
_DBPORT, _DBNAME

SQLALCHEMY_BINDS = {'eg1':'...','eg2':'...'}
SQLALCHEMY_BINDS_SLAVE = {'eg1':'...','eg2':'...'}
```