Manage coc.nvim extensions in dein.nvim

> Since it's installed by vim's plugin manager, it should be updated by that plugin manager.
>
> cf. https://github.com/neoclide/coc.nvim/issues/691#issuecomment-481976822

I tried to add below settings, but not working.

```
[[plugins]]
repo = 'marlonfan/coc-phpls'
on_event = 'BufRead'
on_ft = ['php']
```
