Added --auto flag to configure

Without --auto, ```configure``` will prompt user for download directory
instead of blindly using the default dir (as select already did).

Reason: IMO this makes a better user experience. (This is how I
*thought* how ```configure``` *would* work when I tried downloading
exercises with tmc.py for the first time earlier today. :P)
Merge pull request #15 from aa-m-sa/master

`tmc configure` prompts user for download directory even with --id