Add Arguments for Configuring Params in CLI

[lemorage]

Currently, we need to adjust the value in the config file donkeytype-config.json manually (if we had one). Would it be nice that we can directly set and save these parameters in the CLI. For instance:

$ ./donkeytype config set duration 30
$ ./donkeytype config set uppercase true

Unlike the README examples like ./donkeytype --duration 60 --numbers true --uppercase true for on-the-fly configuration, this new option would allow us to persistently save these settings in the JSON configuration file.

[lemorage]

Also, we can add another subcommand to see all the available config options like so: ./donkeytype config ls.