Improve &rtp management (#85)

- Respect the order of `Plug` commands even when some plugins are loaded
  on demand
- Correct the order of `after` directories added to `&rtp`
Stricter double-comma replacement

To handle paths ending with commas
Merge pull request #88 from junegunn/improve-rtp-mgmt

Improve &runtimepath mgmt (fix #85)