deoplete source: expose relevant asynccomplete configuration options

[bb010g]

Here's a draft mapping from the asyncomplete option set to deoplete:

deoplete#custom#var('tmux-complete', 'splitmode', 'words')
" (Currently a default of v:false for deoplete)
deoplete#custom#var('tmux-complete', 'filter-prefix', v:true)
deoplete#custom#var('tmux-complete', 'show_incomplete', v:true)
" It seems like this could be better named,
" as it removes duplicate matches more than it sorts.
deoplete#custom#var('tmux-complete', 'sort_candidates', v:false)
deoplete#custom#var('tmux-complete', 'scrollback', 0)
deoplete#custom#var('tmux-complete', 'truncate', 0)

It doesn't look like anything is subsumed by deoplete functionality.