Add support for registering Handlebars helpers.

[tonysneed]

Addresses #14, #16.

[tonysneed]

You can register Handlebars helpers in the ScaffoldingDesignTimeServices where setup takes place.

• Create a named tuple as shown with myHelper below.
• Pass the tuple to the AddHandlebarsScaffolding extension method.
• You may register as many helpers as you wish.

public class ScaffoldingDesignTimeServices : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection services)
    {
        // Generate both context and entitites
        var options = ReverseEngineerOptions.DbContextAndEntities;

        // Register Handlebars helper
        var myHelper = (helperName: "my-helper", helperFunction: (Action<TextWriter, object, object[]>) MyHbsHelper);

        // Add Handlebars scaffolding templates
        services.AddHandlebarsScaffolding(options, myHelper);
    }

    // Sample Handlebars helper
    void MyHbsHelper(TextWriter writer, object context, object[] parameters)
    {
        writer.Write("// My Handlebars Helper");
    }
}

• To use Handlebars helper defined above, add the following to any of the .hbs files within the CodeTemplates folder: {{my-helper}}