Add NuGetPreReleaseTag

[roji]

project.json builds allow you to leave the prerelease tag component of the version as a wildcard ("version": "3.1.0-*") and then pass it in via a command-line parameter (dotnet pack --version-suffix).

Ironically with all the variables GitVersion generates, there doesn't seem to be a simple nuget-compatible prerelease tag (i.e. unstable0456 and not unstable.456). Maybe you guys would consider adding NuGetPreReleaseTag?

Related to #647

[JakeGinnivan]

Sounds fine to me.

[qetza]

Done in PR #1002

[stale]

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