Add persistent storage for device data

[postlund]

Short feature/function description Currently pyatv doesn't store any configuration on disk. It would be useful to have simple persistent storage of credentials after pairing, to not have to deal with long command lines. At first this storage can hold credentials and identifiers, but in the future other stuff could be stored as well. So it should be genetic enough to support that.

What needs to be done?

• Add a simple storage module
• Integrate in atvremote
• Update documentation

Is this a breaking change? No

Anything else worth knowing? No