Include the version of `go-github` in `User-Agent` headers sent to the GitHub API

[timrogers]

👋🏻 Hi there! I'm Tim, and I'm a Product Manager at GitHub. To allow us to better understand how people are using our API, it's super helpful to know what versions of SDKs like go-github they are using.

At the moment, this package sends the User-Agent "go-github" in requests, but it doesn't say what version is being used.

This updates the User-Agent header to include that information, whilst still maintaining the ability for people to choose their own user agent by setting the UserAgent field on Client.

It also exports a Version constant, so people can use that information - for example to construct custom User-Agent headers.

This will mean that someone has to update the Version constant with the new version when it is incremented.

This is the first ever Go I've written, so I won't be upset if you tell me that I've done this in a terrible way 😉

[google-cla[bot]]

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[codecov[bot]]

Codecov Report

Merging #2403 (7279ad7) into master (fd22ee9) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2403   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         119      119           
  Lines       10546    10546           
=======================================
  Hits        10342    10342           
  Misses        140      140           
  Partials       64       64           

Impacted Files	Coverage Δ
github/github.go	97.80% <100.00%> (ø)

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[gmlewis]

Thank you, @raynigon ! Merging.