[behance] Add support for collections

[Hrxn]

Support request:

In addition to user profiles and single galleries, Behance also has Collections, which are simply a set of specific projects/galleries on Behance.

I don't know whether I would consider this a "must have" feature, but if this is something pretty straightforward to implement, why not? Maybe take a quick look at it, to see if the existing extractor functionality can be extended easily. You're in the best position to judge, I think.

Some example URLs:

https://www.behance.net/collection/170615607/Sky
https://www.behance.net/collection/150165011/-

The scheme is pretty straightforward, /collection/<id>/<title>.

The title for the second example collection is actually "人像", which is Chinese and not supported in an URL for obvious reasons. Not sure, maybe it is better to convert to safe characters in such cases, or use the ID also as the title here..

[mikf]

The collection title is not important and can even be omitted:
https://www.behance.net/collection/170615607 gets treated the same as https://www.behance.net/collection/170615607/Sky when putting it into gallery-dl.

Browsing https://www.behance.net/collection/170615607 gets you a 404, but you can put anything as a title and it still works: https://www.behance.net/collection/170615607/abc