Bootstrap updates

Adds new features to the bootstrapping file:

* -f, --fullscreen:  force fullscreen mode
* -w, --windowed:  force windowed mode
* --resolution WxH:  force the given resolution, coerced from a string
such as "1280x800" (valid resolution components are between 100 and
99999)
* -H, --headless:  ignore creation of application window (also ignores
previous switches)

Analogous parameters have been added to Arpk.start_app() and
Arpk::App.new()