This exposes the current version of the `go-github` package
(e.g. `"45.2.0"`) as a field on the `Client` struct.

This will allow users of `go-github` to access the current version
of the package, for example if they want to construct a custom
`User-Agent` header including it.

This change means that, before releasing tagging and releasing a
new version of the package, this string constant will have to be
updated.

… header

This updates the default request header sent by the module to
include the current version, e.g. `go-github/45.2.0`.

This makes it easier for GitHub to track usage of this SDK and
how users are upgrading between versions.

Users can continue to overwrite this default user agent with
their own string by setting the `UserAgent` property on the
`Client`.