[Mining] MineToAddress parameter in QT and CLI miner

[adilwali]

As of right now, a user must transfer their wallet.dat to any machine they want to mine on. Or, perhaps, differently stated, they will must secure the wallet.dat on all machines where they mine.

This is not quite ideal. And makes the context of mining a little more stressful and possibly less safe.

We could remedy this by simply adding an optional 'minetoaddress' parameter in setiming, so that the command might look like like:

merit-cli setmining true <ToAliasOrAddress> 4 10 100

We could also make a separate command, if that is more sensible. That might look like:

merit-cli setminingtoaddress <ToAliasOrAddress> 4 10 100