Return keys from commands

[ofekshenawa]

No description provided.

[vladvildanov]

This approach isn't scalable, each time we need a new type of extracting we have to add a command here. Cmder is an interface that should be abstract as possible and should not contain any implementation specific (mget is an implementation specific in this case).

Cmder should return a keys, but shouldn't be aware about the keys position of specific command (this is implementation details), instead the specific command that implements this interface should define keys position for itself.