Add ReportGenerator helper

[MarcoRossignoli]

contributes to #387

Added report generator helper, in this way we can generate during debugging and testing full report to better understand if results are ok.

cc: @tonerdo @petli

@danielpalme I saw that report generator supports plugin...I would like "run generator" from my code base(adding core package) can you point me to some code sample?
I don't wan to create a custom report, only run, for instance ReportGenerator.Run(reportfile,reportdirectory)
BTW code below works well, but we could avoid global tool install.

[danielpalme]

@MarcoRossignoli
The following approach should work for you:

1. Install package ReportGenerator.Core
2. Use one of the following code snippets:

Palmmedia.ReportGenerator.Core.Program.Main($"-reports:\"{reportFile}\" \"-targetdir:{dir.FullName}\" {(string.IsNullOrEmpty(sourceFileFilter) ? string.Empty : $" -filefilters:{sourceFileFilter}")}");

new Palmmedia.ReportGenerator.Core.Generator().GenerateReport(new Palmmedia.ReportGenerator.Core.ReportConfiguration(
    new[] { reportFile },
    dir.FullName,
    new string[0],
    null,
    new string[0],
    new string[0],
    new string[0],
    new string[0],
    string.IsNullOrEmpty(sourceFileFilter) ? new string[0] : new[] { sourceFileFilter },
    null,
    null));

[MarcoRossignoli]

Great thank's!