[#140] add simple completion function

[Quramy]

provide tsuquyomi#getCompletion function for #140.

[Quramy]

@prabirshrestha

You can create asyncomplete#sources#tsuquyomi#completor with tsuquyomi#getCompletion function.

I've checked the operation via the following code:

function s:ts_comp(opt, ctx)
  call tsuquyomi#getCompletions({candidates, startcol ->
         \ asyncomplete#complete('typesctipt', a:ctx, startcol, candidates)
         \ })

endfunction

call asyncomplete#register_source({
    \ 'name': 'tsuquyomi',
    \ 'whitelist': ['typescript'],
    \ 'priority': 5,
    \ 'completor': function('s:ts_comp'),
    \ })

How about it ?

[prabirshrestha]

If the server has not started is it still considered dead?

[prabirshrestha]

looks good but some comments here.

• asyncomplete#complete('typesctipt', should beasyncomplete#complete('tsuquyomi', since that is the name you used.
• rather than checking if tsuquyomi#statusServer() == 'dead' is it possible to know if the tsserver has been initialized instead. Initializing tsserver for large code base is slow. so the first time I type vim in unresponsive for few seconds since tsuquyomi is not completly async and then once it has been initialized the completion is fast.

[prabirshrestha]

Seems like even saving a file for a big project has this issue. So I don't think anything can be done now to fix the initial unresponsiveness unless it is moved to full async implmentation.

PR looks good.