Support running the tool on a csproj that does not have version yet

[pingvinen]

Since dotnet new classlib does not add <Version> to the csproj file, it becomes a manual process to add the version tag before being able to use dotnet version for the first time.

I think it would make sense to support the case and then defaulting the version to 0.0.0 (before bumping) and perhaps output a warning that there was no version at it was therefore defaulted to 0.0.0 before applying the bump.

I do not think it would be necessary to detect a version set using assembly attributes, as that is basically a different style (and in my opinion an obsolete one with Core) of supplying the version.

[nover]

Should be fixable just around here in the code:
https://github.com/skarpdev/dotnet-version-cli/blob/master/src/CsProj/ProjectFileParser.cs#L37