Load plugins only once in plug#load (#616)

When loading 'deoplete.nvim' for the 2nd time during InsertEnter
manually, the `s:dobufread` (or `s:lod` itself) prevents it to work
properly - likely because the plugin gets resourced.

Maybe there could be a way to force this (and reload plugins always),
but by default it seems to make sense to skip already loaded plugins.

plug.vim

@@ -447,11 +447,15 @@ function! plug#load(...)
     let s = len(unknowns) > 1 ? 's' : ''
     return s:err(printf('Unknown plugin%s: %s', s, join(unknowns, ', ')))
   end
-  for name in a:000
-    call s:lod([name], ['ftdetect', 'after/ftdetect', 'plugin', 'after/plugin'])
-  endfor
-  call s:dobufread(a:000)
-  return 1
+  let unloaded = filter(copy(a:000), '!get(s:loaded, v:val, 0)')
+  if !empty(unloaded)
+    for name in unloaded
+      call s:lod([name], ['ftdetect', 'after/ftdetect', 'plugin', 'after/plugin'])
+    endfor
+    call s:dobufread(unloaded)
+    return 1
+  end
+  return 0
 endfunction
 
 function! s:remove_triggers(name)