can setting tabnine's min and max value

[get-me-power]

Hi, @kitagry ! Thanks for creating nice vim-plugin!

I used global variables to allow some values to be set on a per-user basis. Could you check this PR?

[get-me-power]

I fixed conflict.

[kitagry]

Thanks @kazukazuinaina

But, I would like to set these variables like followings.This is same as asyncomplete-buffer.vim. So, could you fix it like this?

call asyncomplete#register_source(asyncomplete#sources#tabnine#get_source_options({
  \ 'name': 'tabnine',
  \ 'allowlist': ['*'],
  \ 'completor': function('asyncomplete#sources#tabnine#completor'),
  \ 'config': {
  \   'line_limit': 1000,
  \   'max_num_result': 20,
  \ }))

[get-me-power]

@kitagry Thanks for review. How about this changing?

[kitagry]

Thanks @kazukazuinaina

[get-me-power]

@kitagry Thanks for your merging!