Add support for meta programming

[davidfowl]

Everything under Compilers/Preprocess/*/.cs will become an independent assembly. The assembly needs to implement an ICompileModule:

[AssemblyNeutral]
public interface ICompileModule
{
    void BeforeCompile(IBeforeCompileContext context);
    void AfterCompile(IAfterCompileContext context);
}

[AssemblyNeutral]
public interface IBeforeCompileContext
{
    CSharpCompilation CSharpCompilation { get; set; }
    IList<ResourceDescription> Resources { get; }
    IList<Diagnostic> Diagnostics { get; }
}

[AssemblyNeutral]
public interface IAfterCompileContext
{
    CSharpCompilation CSharpCompilation { get; set; }

    IList<Diagnostic> Diagnostics { get; }
}

The current compilation context will be passed into the process method and a compilation will be expected in return.

• This will be called at runtime, build time and design time (in the DTH)
• It will use the application's dependencies as well as development dependencies (see Add support for development dependencies #38)
• This is the same interface that will be discovered when you want to write a nuget package that wants to meta program assemblies. For example the razor precompiled views will be a standalone nuget package that will be able to turn razor files into C# files precompiled as part of the application assembly.