Add an option to limit the number of parallel connections about bandcampdownloader

[ajsnyde]

With the current version, there is an exponential back-off strategy that limits the spammed attempts. Not sure if this alone is sufficient. I'll check into how easily the feature could be added, and might work something out.

[Otiel]

Closing this as the existing exponential back-off mechanism seems to be enough to prevent tripping the Bandcamp anti-spam filter.

Please comment if you feel it could still be of use.