Support database selection

[rdohms]

In order to allow the use of different databases in Redis, the
connection now supports switching to a new database.

One important factor to understand here is that if you are using
pconnect and you do a select operation, you may be changing the same
connection the rest of your code is using. Due to this its recommended we merge #87
along with this one to avoid weird scenarios.

[haiwenzhu]

function flushRedis in Storage/Redis.php need use flushDb instead of flushAll when use database option, or all database data would be flushed.

[nstapelbroek]

Thanks @rdohms, because @renatomefi demanded I'll ask around if we can merge this 🌚