NuGet isn't strictly SemVer

Similar to #5 (thank you so much for fixing it!!), it looks like NuGet is also assumed to be SemVer: https://github.com/nexB/univers/blob/21864169341c4949bd40c931cd8450488de24d64/src/univers/versions.py#L266

However, [NuGet's documentation](https://docs.microsoft.com/en-us/nuget/concepts/package-versioning#where-nugetversion-diverges-from-semantic-versioning) shows list important differences:

> NuGetVersion supports a 4th version segment, Revision, to be compatible with, or a superset of, [System.Version](https://docs.microsoft.com/en-us/dotnet/api/system.version). Therefore, excluding prerelease and metadata labels, a version string is Major.Minor.Patch.Revision. As per version normalization described above, if Revision is zero, it is omit from the normalized version string.
>
> NuGetVersion only requires the major segment to be defined. All others are optional, and are equivalent to zero. This means that 1, 1.0, 1.0.0, and 1.0.0.0 are all accepted and equal.
>
> NuGetVersion uses case insenstive string comparisons for pre-release components. This means that 1.0.0-alpha and 1.0.0-Alpha are equal.

The current implementation does not follow these rules:
```
> NugetVersion('1.0.0.0') < 'NugetVersion('1.0.0.1')
> False
```

```
>>> NugetVersion('1.0.0-Alpha') == NugetVersion('1.0.0-alpha')
False
```

Nuget's implementation appears to be here: https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Core/NuGet.Versioning/NuGetVersion.cs