Add support for Analyzers with Configuration

[manfred-brands]

Fixes #272

I choose for adding AnalyzerConfig to Settings, instead of to every overload of RoslynAssert.

Instead of adding new overloads for Settings constructor and CodeFactory.CreateSolution, I decided to add an extra optional parameter. If you prefer the other way using overloads, let me know and I will update the PR.

A few quirks of Roslyn API:

• An AnalyzerConfigDocument must be rooted, i.e. start with a /. AnalyzerConfig.cs
• A source document (DocumentInfo) must have a file name, either direct or implied from the ProjectInfo otherwise its configuration is set to None. See: ProjectState.cs

[JohanLarsson]

Very nice!

[JohanLarsson]

Release nuget now or do you have more things lined up?

[manfred-brands]

Now would be good. Thanks.