Add a subcommand to generate an example config

[dontlaugh]

It would be convenient to be able to do something like

slack-term config-example > ~/.slack-term

With blank values for secrets, of course. This would make distribution a bit easier and more
script friendly.

[jpbruinsslot]

I'm not so keen on adding a separate command for a config file creation for one single value. However, I might consider the creation of the .slack-term file when starting the application for the first time.