Add quick config manipulation command 

Hi there!

First of all thanks for your awesome work! I'm evaluating cloudControl as our new PaaS and what i saw looks downright great so far.

However, I struggled a little with the custom config addon. What if i want to change or add a setting? Ok, i can remove the addon and reenable it with updated values. The other option is to write a quick cli command which is what this PR contains:

    $ cctrlapp app/default config.set SOME=THING
    SOME                : THING

    $ cctrlapp app/default config.set ANOTHER=ONE
    SOME                : THING
    ANOTHER             : ONE

    $ cctrlapp app/default config.set SOME=say
    SOME                : say
    ANOTHER             : ONE

    $ cctrlapp app/default config.get SOME
    say

    $ cctrlapp app/default config.unset SOME
    ANOTHER             : THING

    $ cctrlapp app/default config --shell
    ANOTHER=THING


Yes, it's exactly how another cli by a popular PaaS behaves ;-). 

Please let me know if you think this is a good idea.

PS: One considerable issue with this one is that an app needs to be restarted after updating the config. 
I assume that is what deploy is for, right?

PPS: If you guys change your APIs so that addon options can be changed this PR is considered closed