WithRawArgument(string) extension method on ToolSettings

### Problem

There are times when I'm either debugging a Cake script or using an incomplete settings object that I want to customize the arguments, but I don't want to use the longer syntax `settings.ArgumentCustomization = a => a.Append("--some-argument")`.

The longer syntax also does not compose well. For example, let's say I already have this code and now I need to test how it runs if I add `--some-argument`. It doesn't end up being a simple and quick:

```cs
var settings = new NUnit3Settings { … };

if (parameters != null && parameters.Count != 0)
{
settings.ArgumentCustomization = a =>
{
foreach (var parameter in parameters)
a = a.AppendSwitch("-p", EscapeProcessArgument(parameter.Key + '=' + parameter.Value));

return a;
};
}
```

### Suggestion

What if we had a terse, composable helper do this?

```cs
var settings = new NUnit3Settings { … }
.AddRawArgument("--some-argument");

if (parameters != null)
foreach (var parameter in parameters)
settings.AppendRawArgument("-p=" + EscapeProcessArgument(parameter.Key + '=' + parameter.Value));
```

Here's what I end up adding to Cake scripts:

```cs
public static T WithRawArgument(this T settings, string rawArgument)
where T : Cake.Core.Tooling.ToolSettings
{
if (settings == null) throw new ArgumentNullException(nameof(settings));

if (!string.IsNullOrEmpty(rawArgument))
{
var previousCustomizer = settings.ArgumentCustomization;
if (previousCustomizer != null)
settings.ArgumentCustomization = builder => previousCustomizer.Invoke(builder).Append(rawArgument);
else
settings.ArgumentCustomization = builder => builder.Append(rawArgument);
}

return settings;
}
```

That's just a straw man, but would you consider something similar?

Start at Cake.Core.Tooling.ToolSettings and its ArgumentCustomization property, then compare the proposed WithRawArgument extension with the existing argument-builder behavior shown in the issue. Determine the final composable API and add coverage for empty arguments, existing customizers, and returning the same settings object; done means raw arguments can be appended without replacing earlier customization.

Written by the indexing model from the issue text.