Convert setting parameters to flags

[kasnerz]

Some settings would be easier to use as command line flags instead of parameters in config.yml.

Currently these are cache_dev_splits and hostname (i.e. the ones which differ during deployment).

The implementation needs to take into account that run in fact calls the default flask run command - I was not able to find a way to integrate extra parameters into this command.

But it should be possible to implement it as tabgenie [EXTRA_ARGS] run [FLASK_ARGS] (although it is a bit clumsy).