feat: channels followed & followers

[jackmcguire1]

• add new followed channels endpoint

• add new channel followers endpoint

• add unit tests

• go fmt fixes

• https://dev.twitch.tv/docs/api/reference/#get-followed-channels

• https://dev.twitch.tv/docs/api/reference/#get-channel-followers

[coveralls]

Pull Request Test Coverage Report for Build 4091716952

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 93.167%

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Totals
Change from base Build 4038132135:	0.1%
Covered Lines:	1459
Relevant Lines:	1566

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💛 - Coveralls