Release helper
Various users complained that Nimble is sometimes too picky around repository structure and sometimes too lax (i.e. ignoring files that should be installed)
I suggest introducing a `nimble upload-release <x.y.z>` command that:

- Check if x.y.z is greater than the current version in the .nimble file
- List files that will be installed (sorted)
- List files that will not be installed, with a warning
- Optionally list files that are not tracked in git/hg and will not be installed
- Update the .nimble file with the new release number
- Tag the release on git/hg unless `--notag` is passed

Related #561 #582 #536