This commit adds [NerdBank.GitVersioning](https://www.nuget.org/packages/Nerdbank.GitVersioning), which gives every build of plist-cil a unique version number.
The base version number is stored in `version.json` (e.g. 2.2) and is incremented by 1 every time a commit is added. Builds off branches contain a `+g{commit}` suffix, indicating that these builds are prerelease.
This makes it easier to keep track of different versions of plist-cil.
