Add ReSharper attributes

[Tyrrrz]

Almost all code paths in YoutubeExplode never return null, yet users might not know about it and surround their code with null propagations or checks.

Adding ReSharper attributes should help a bit with that.

[Tyrrrz]

I decided this is too much unnecessary work for little benefit. Plus people who paranoically check everything for null probably don't use ReSharper anyway.

[Tyrrrz]

Changed my mind again