Deal with glyphs w/o assigned script (e.g. Punctuation)

[ohbendy]

Allow glyphs with no script assignment (e.g punctuation ( ) + - % : ; etc) to have mark zones. I tried using 'None', 'No value' and 'Common' in the custom parameter but those glyphs still wouldn't show any zones.

[Mark2Mark]

might be linked with #9