Skin proposals

[xzfc]

1. Skins interface with __getitem__ method that returns ether Null or skin URI.
   1a) Same as above, but returns named tuple (str skinUri, bool displayName). Some skins (8, nasa, politicians) don't have to display names.
2. AgarSkins that implements Skins. On initialization it fetches http://agar.io/ and grabs actual skin list.
3. AgariomodsSkins that fetches list (from where?) or just have hardcoded list.
4. ImgurSkins that handles i/... skins.

[Gjum]

Premium and agariomods skins have been implemented in a fork. I'd merge that if they do a PR.