Proposal: Move methods off of Client

[WillAbides]

Tooling like update-urls and metadata could be simpler if API calls were made from methods on a *Service receiver. I think it would also make more sense for humans using go-github.

Currently there are handful of Client methods such as that would logically belong on different receivers. I propose we create the appropriate Service types for these.

The changes would be would be:

• Client.Markdown -> MarkdownService.Markdown (maybe rename it to RenderMarkdown while we are at it)
• Client.ListEmojis -> EmojisService.ListEmojis
• Client.ListCodesOfConduct -> CodesOfConductService.ListCodesOfConduct
• Client.GetCodeOfConduct -> CodesOfConductService.GetCodeOfConduct
• Client.APIMeta -> MetaService.APIMeta
• Client.Octocat -> MetaService.Octocat
• Client.Zen -> MetaService.Zen

The original methods would be updated to call their new equivalent and marked deprecated.

[WillAbides]

Or we could create a MiscService and put them all in there.