Add tests

[jbheard]

It would be nice to have tests for things. This will be especially important if we start adding more complex patterns like analyzers.

[jbheard]

Something nice to go with this would be to move all complexity out of Program.cs to make things more testable.

Then the main should look something like:

public static async Task<int> Main( string input, string output ) {
    var worker = new MarkdownFileWorker( input, output );
    await worker.DoWorkAsync();
}