Run single detector

This is the implementation of a long-standing feature request: #13.

During the implementation, I came across some design issues with the current code. I'd like to pick these up next (see FIXMEs and TODOs in this pull request).
But this will always be a list with just one item, won't it?
I mean, we talked about running one single smell detector, **not** multiple ones.

 Shouldn't then @smells_to_detect rather be @smell_to_detect and not a list, but just string?