made page stack optional, added default stack

since you can now toggle default stack, or disabled page stack entirely,
i added ?draft querystring to be used in those cases. This allows you to
use your own clutterless interface for drafts editing if its not used
much, or enabled only in certain instances.

'GSUSEDRAFTS'          => true,                           // (bool) use
page drafts
'GSUSEPAGESTACK'       => true,                           // (bool) use
page stacks for drafts, else `nodraft` or `draft` only
'GSDRAFTSTACKDEFAULT'  => true,                           // (bool)
default page stack editing to drafts if true