[Feature Request] Support Conditional Requests to reduce github api usage.

[KaelBaldwin]

go-github supports github API's conditional requests, but it requires you to use your own Transport that is set up for it.

In one of my own projects that uses go-github, I would create an http.Client and set it's Transport to one created using https://github.com/gregjones/httpcache

I would then attach that Client to a context using it's WithValue method.

In order to support the various caching methods, it would be nice if we could pass in a Context object when setting up go-github-selfupdate, that way we could implement Conditional Requests on our end when using it.