Allow customizing generated tag name

[ronaldbarendse]

Currently running nbgv tag will always tag the current commit using a fixed v{version} format:

Nerdbank.GitVersioning/src/nbgv/Program.cs

Line 589 in 4218cad

string tagName = $"v{oracle.SemVer2}";

Not everyone will use this format though (it's common to omit the v prefix or use release- instead), so making this configurable would allow using this command to automatically tag the correct version using their preferred/already in-use format.

This could be added in a similar way to the current release.branchName setting (and maybe also within the same release section), e.g.:

{
  "release" : {
    "tagName" : "v{version}"
  }
}