Add "games" tag to game-related search engines

[rpdelaney]

Description

• Tag "steam" provider with "games"
• Tag "twitchtv" provider with "games"
• Tag "unity3d" provider with "games"

I did not see any other existing providers that return content that is exclusively game related.

Based on some limited local testing I feel like "unity3d" is probably out of place here, but I included it so you can make the call. If you agree I will revert that one, or you can feel free to make changes to my PR.

Checklist

• [x] All files have been linted with gofmt.
• [x] Documentation has been updated.
• [x] All tests pass.

[zquestz]

Have to agree, that unity3d doesn't really fit. Lets not tag that one.

[rpdelaney]

Ok, reverted in c9b127b