List available engines from the CLI

[TomV]

Currently the only way to know what engines are supported by nodejitsu is to search the documentation or ask in irc.. I've been waiting for support for 0.10.x for a while now, and every now and then I need to search and see if it's supported.

Would be nice if it was as simple as

>>jitsu --engines
{ 
    supported    :  ["0.6.22","0.8.19", "0.8.25" ] 
  , deprecated   :  []
, experimental   :  ["10.12"] 
}

Then update the docs to explain how to get the currently supported engines, rather than needing to update the docs any time you update the engine support.

[edef1c]

This is on our roadmap.