Add pandoc syntax support

[apuignav]

Since pandoc support is getting better and better, it is quite commonly used to write markdown. Extensions such as vim-pandoc/vim-pandoc introduce a new file type (and syntax) called pandoc. To get some syntax highlighting in zenroom2 from the vim-pandoc/vim-pandoc-syntax plugin, I have added a new function that is loaded only if the filetype is pandoc.