Pretty printing extensible model.

[ManishJayaswal]

Add better stack trace printing support as an example using the extensibility

[ManishJayaswal]

@genlu

[amcasey]

Presently, csi uses Microsoft.CodeAnalysis.Scripting.Hosting.InteractiveScriptGlobals.Print.

[amcasey]

Microsoft.CodeAnalysis.Interactive.InteractiveHost.Service also uses Microsoft.CodeAnalysis.Scripting.Hosting.InteractiveScriptGlobals.Print.

[amcasey]

InteractiveScriptGlobals.Print delegates to the public abstract type Microsoft.CodeAnalysis.Scripting.Hosting.ObjectFormatter, which appears to have been designed as an extension point. It looks like we might just need to expose the members of ObjectFormatter.Formatter as extension points and then provide a way to swap out the ObjectFormatter without recompiling (e.g. MEF).

[amcasey]

Specifying user-defined analyzers at compile-time seems like an analogous problem. I'll investigate how those are picked up and maybe we can do the same thing with "formatter" assemblies.

[amcasey]

Analyzers are tagged with an attribute and then discovered using a MetadataReader. Once the fully qualified type names are known, the objects are instantiated via reflection.

[amcasey]

I think we're going to start with a simpler model where the host object has a read-write ObjectFormatter property. I've made the change locally and it seems to "just work".

[amcasey]

We scaled this back to user configurability in 1.2. A potential API is in place but is currently internal.