Disable 'shellslash' on Windows to avoid problems

Probably fix #54

autoload/vimwiki/base.vim

@@ -466,7 +466,18 @@ function! vimwiki#base#system_open_link(url) "{{{
   " handlers
   function! s:win32_handler(url)
     "http://vim.wikia.com/wiki/Opening_current_Vim_file_in_your_Windows_browser
-    execute 'silent ! start "Title" /B ' . shellescape(a:url, 1)
+    "disable 'shellslash', otherwise the url will be enclosed in single quotes,
+    "which is problematic
+    "see https://github.com/vimwiki/vimwiki/issues/54#issuecomment-48011289
+    if exists('+shellslash')
+      let old_ssl = &shellslash
+      set noshellslash
+      let url = shellescape(a:url, 1)
+      let &shellslash = old_ssl
+    else
+      let url = shellescape(a:url, 1)
+    endif
+    execute 'silent ! start "Title" /B ' . url
   endfunction
   function! s:macunix_handler(url)
     execute '!open ' . shellescape(a:url, 1)