Add g:vimwiki_create_link option to prevent link creation.

Adds a new global option named create_link, which allows the user to
control :VimwikiFollowLink behavior when attempting to follow a
non-existent link. The original behavior of creating a new link is
preserved. However, by setting this option to 0, new links will not be
created when pressing return over regular text. Closes #528.

autoload/vimwiki/base.vim

@@ -1200,7 +1200,7 @@ function! vimwiki#base#follow_link(split, ...)
   else
     if a:0 >= 3
       execute "normal! ".a:3
-    else
+    elseif vimwiki#vars#get_global('create_link')
       call vimwiki#base#normalize_link(0)
     endif
   endif