Allow setting theming options from OCC

[rullzer]

For large scale deployments it would be great to have the ability to automatically set themeing from the command line.

CC: @juliushaertl

[juliusknorr]

Setting the values is partly possible with occ config:app:set, but we need a proper way to clear the cache afterwards. Also setting images in not possible atm, so I guess adding a simple occ command is the best way to handle this.