Add VK.com

[lolmaus]

VK (old name VKontakte, "in contact") is a very active social network of Russian origin, it's audience size is ~5% of that of Facebook, which is still incredibly huge.

API docs: https://new.vk.com/dev/share_details

[olets]

@mxstbr do you want to limit networks to international ones?

@lolmaus check the contribution notes here https://github.com/mxstbr/sharingbuttons.io/wiki

[mxstbr]

I'm fine with VK since it's huge, I think we could just not enable it by default?

(Also, that contribution guide is awesome! Thanks!)