Add verbose output throughout commands

We have a global `--verbose` flag, but at the moment it's not doing much.

In order to make it vastly more easy to debug stuff, I'd like to add verbose output at every step of the way throughout the download, submit, and configure commands.

To add verbose output:

* import "github.com/exercism/cli/debug"
* at each step, add `debug.Printf()` statements

This should also set debug output in the packages that the commands use, e.g. the `api`, `comms`, `config`, and `workspace`.