Added support for predefined abbreviations.
Added support for predefined abbreviations.

filipstefansson/bootstrap-3-shortcodes,mariokonrad/php-markdown,c9s/php-markdown,jockchou/php-markdown,TypeFriendly/PHP-Markdown,n054/php-markdown,MWDelaney/bootstrap-3-shortcodes,MWDelaney/bootstrap-3-shortcodes,itafroma/php-markdown,nazar-pc/php-markdown-next,robbieaverill/php-markdown,filipstefansson/bootstrap-3-shortcodes,MWDelaney/bootstrap-3-shortcodes,queirozfcom/php-markdown-with-syntax-highlight,alganet/ElephantMarkdown,Kapeli/php-markdown,ashur/php-markdown