Add option for ProcessSettings to opt out of working directory magic

[patriksvensson]

When starting a process from Cake I encountered a problem.

    context.StartProcess("autorest.cmd", new ProcessSettings {
        Arguments = new ProcessArgumentBuilder()
            .AppendSwitchQuoted("--input-file", "=", input)
            .Append("--add-credentials")
            .Append("--csharp")
    });

Since autorest.cmd works with the working directory, and Cake set the process' working directory to the current one (where we execute the build script), autorest.cmd will try to find it's dependencies such as ``node_modules` in the script directory which will fail.

Suggestion:
I think we should add an option to not use the working directory at all. This would make it possible to opt out from the behavior while still not break any other applications.