wordcount: Word count for the combined file types
wordcount: Word count for the combined file types

[Pandoc flavoured Markdown](https://en.wikipedia.org/wiki/Markdown#CommonMark) has its [own file type in `vim`](https://github.com/vim-pandoc/vim-pandoc-syntax#standalone), namely `markdown.pandoc`.
Word count by default in `vim-airline` obviously would make a lot of sense for this popular file type.

Fix this, by making sure that the wordcount extensions will apply
correctly if any of the combined filetypes (e.g. either pandoc or
markdwon) matches the list of allowed filetypes.

chrisbra/vim-airline,yzhou61/vim-airline,rchicoli/vim-airline,bling/vim-airline