[FEATURE] Add cache
[FEATURE] Add cache

Improve speed by adding cache to function parseCssDeclarationBlock.
ricardofiorani/emogrifier,OzzyCzech/emogrifier,Synchro/emogrifier,iFixit/emogrifier,sanderkruger/emogrifier,jjriv/emogrifier,Seldaek/emogrifier,MyIntervals/emogrifier,ernsto/emogrifier