vimwiki-test with wiki-external links

index.mdw

# redongh markdown vimwiki test

* vimwiki-defs my `.vimrc`:
```
let g:vimwiki_global_ext = 0
let g:vimwiki_list = [{
\ 'path': '~/vimwiki/',
\ 'path_html': '~/vimwiki.out.html',
\ 'custom_wiki2html': '~/vimwiki.helpers/convert.py',
\ 'syntax': 'markdown',
\ 'ext': '.mdw',
\ }]
```

* override in my `.vimrc`:
```
function! VimwikiLinkHandler(link)
  if a:link =~# '^file:'
    try
      execute ':split ' . a:link[5:]
      return 1
    catch
      echo "Failed opening file-link in vim."
    endtry
  elseif a:link =~# '^raw:'
    try
      execute ':split ' . '~/' . a:link[4:]
      return 1
    catch
      echo "Failed opening raw-link in vim."
    endtry
  endif
  return 0
endfunction
```

* convert.py as used by gpanders on [comment477815553](https://github.com/vimwiki/vimwiki/issues/642#issuecomment-477815553)
* a common [internal-link](internal-link)
* a file link to [external-txtfile](file://$HOME/testsubdir/external.txt)
* a raw link to [external-txtfile](raw:testsubdir/external.txt)

test-contribution for vimwiki issue1271