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

This is how I (as an user) would prefer the ```tmc configure``` to work (and thought what ```configure``` *would* do), i.e. prompt the user for the download directory, even if user specifies  the course ```--id```. The old behavior (using the default ```~/tmc/course``` directory) can be enabled with ```--auto``` flag. 

I can imagine many other sensible ways of implementing a similar functionality (e.g. providing ```--path``` argument to allow user to specify the download dir) you might prefer instead; I'm making this pull request just to give an idea :-) .

If there's some reason (that I didn't realize) why ```configure``` shouldn't work like this, alternatively please update the documentation to at least note that if the course is selected with ```configure --id```, tmc.py will use the default ```~/tmc/course``` directory.

(Apologies for the slightly botched up commit history.)