Redact API key from debug output.

[Tonkpils]

Also adds '--full-api-key' flag to print out the full API key if needed
Fixes #357

Sample output

./exercism debug

**** Debug Information ****
Exercism CLI Version: 2.3.0
Exercism CLI Latest Release: 2.3.0
OS/Architecture: darwin/amd64
Build OS/Architecture darwin/amd64
Home Dir: /Users/leo
Config File: /Users/leo/.exercism.json
API Key: 9d19*************************996
Exercises Directory: /Users/leo/code/exercisms-dev
Testing API endpoints reachability
	* API: http://localhost:4567 [Get http://localhost:4567: dial tcp [::1]:4567: getsockopt: connection refused] 2.561129ms
	* GitHub API: https://api.github.com/ [connected] 318.750813ms
	* XAPI: http://x.exercism.io [connected] 653.905343ms

[kytrinyx]

Niiice! Thanks @Tonkpils