Add wiki.vim file opening scheme for pdf, html

Can open file-links that the cursor is currently over (in markdown link
format). PDF is opened in zathura, html in qutebrowser, the rest in vim.

nvim/.config/nvim/plugin/wiki.vim

@@ -39,3 +39,16 @@ endfunction
 let g:wiki_link_target_map = "PrependDatestamp"
 
 let g:wiki_list_todos = ['[ ]', '[x]']
+
+" set file opening schemes
+function! WikiFileOpen(...) abort dict
+  if self.path =~# '.pdf$'
+    silent execute '!zathura' fnameescape(self.path) '&'
+    return 1
+  elseif self.path =~# '.html$'
+    silent execute '!qutebrowser' fnameescape(self.path) '&'
+    return 1
+  endif
+  return 0
+endfunction
+let g:wiki_file_open = 'WikiFileOpen'