Convert all maps and commands to use syntax information.

[cirosantilli]

Lot's of information is currently duplicated on the commands.

E.g., to find the next header, ]] redefines the header regexp at: https://github.com/plasticboy/vim-markdown/blob/9ced6869ac811055068cf1dfb9d6904a385fcfeb/ftplugin/mkd.vim#L49

All those functions should instead reuse the existing syntax highlight. This can be retrieved for example with:

synIDattr(synID(line('.'),col('.'),1),'name')

Also see: http://stackoverflow.com/questions/1467438/find-out-to-which-highlight-group-a-particular-keyword-symbol-belongs-in-vim