Extract common Library to enable consumption from other tools

I want to write a Cake addin for MiniCover like the one that exists for OpenCover. I cannot easily instruct users to add MiniCover as a tool as the way cake downloads tools from Nuget is not compatible with the DotnetCliTool package type. I could tell users to create a tools project as instructed in the README and pass that to my addin, but I think an easier solution is if we separate the actual instrumentation and reporting logic from the dotnet cli tool frontend. This would allow me to reference it as a dependency in my package and enable users to get simple coverage with something like:

#addin "Cake.MiniCover"

// ...

Task("Coverage")
    .IsDependentOn("build")
    .Does(() => 
{
    MiniCover(tool =>
        {
            foreach(var project in GetFiles("./test/**/*.csproj"))
            {
                tool.DotNetCoreTest(project.FullPath, new DotNetCoreTestSettings()
                {
                    // Required to keep instrumentation added by MiniCover
                    NoBuild = true,
                    Configuration = configuration
                });
            }
        },
        new MiniCoverSettings()
            .WithAssembliesMatching("./test/**/*.dll")
            .WithSourcesMatching("./src/**/*.cs")
            .GenerateReport(ReportType.CONSOLE | ReportType.XML)
    );
});

// ...

Then, generating coverage becomes as simple as

./build.sh -t Coverage

# Or on Windows:

./build.ps1 -t Coverage

[ffMathy]

I love it! 😍

[day01]

@nlowe Looks really good. @lucaslorentz do you plan to create a project on github? That would be nice to have kanban with tasks. Maybe the common board with Cake.MiniCover (@nlowe)?