NPM script to generate beta packages

[fluffynuts]

Beta nuget packages have a version appendage, eg -r1, or the more descriptive -{datestamp}-{commit_sha}; zarro already supports generating packages in this format, so actually producing the artifact shouldn't be that hard. However, the artifact should also be pushed to a repository somewhere - either to nuget (least surprise for users) or GitHub (to keep them separate from official releases, if that's even a desirable thing).

Ideally, we should produce a beta package any time code is pushed to GitHub in this repo so that it can be tested in consuming projects - and hopefully if we're making changes to resolve issues, we could point a user at a beta package to verify that the issue has indeed been resolved.