Allow specify a different app.settings file at runtime

[vasily-kirichenko]

I really don't think it's useful. App.config is always automatically transformed to <exe name>.exe.config.

[smoothdeveloper]

It has use when you want to use it from .fsx and pass arbitrary configuration file, assuming #63 is implemented.

[Thorium]

Now you can use this from F# interactive, so this could be closed...