move nvim-completion-manager Ultisnips source into Ultisnips repository.

[roxma]

Hi, I've been working on an async completion framework for neovim, and it also woks on vim8.
https://github.com/roxma/nvim-completion-manager

As the framework API has been stable, I think it would be nice to move UltiSnips completion source into ultisnips git repository.

This PR adds minimized overhead to ultisnips, it only register a single autocmd at startup.

It also simplifies the code so that it doesn't need to check the installation of ultisnips any more.

I'll handle further issue on this source, and send PR if necessary.

[roxma]

For anyone interested on how nvim-completion-manager co-operate with ultisnips, here's a screencast copied from roxma/nvim-completion-manager#12 (comment)

There's also another all in one screencast which shows how the snippet list is auto popped up by NCM.

---

Triggering Ultisnips or show popup hints with the same key <c-u>

When the expand key is pressed and nothing has been typed, a popup list for snippets will be triggered.

let g:UltiSnipsExpandTrigger = "<Plug>(ultisnips_expand)"
inoremap <silent> <c-u> <c-r>=cm#sources#ultisnips#trigger_or_popup("\<Plug>(ultisnips_expand)")<cr>

[roxma]

Sorry for the bothering, I just found a better way to detect ultisnips installation,

It is still ok to keep this in NCM's repository, and It could be one of the simplest example for writing NCM source.

I'm closing this PR. Sorry for the bothering, again.