Allow fzf#complete to take hash argument with sink

This allows us to use the return value of fzf#wrap function with
fzf#complete.

This commit also removes obsolete g:fzf#vim#default_layout and
fzf#vim#layout.

doc/fzf-vim.txt

@@ -274,7 +274,7 @@ following exceptions:
    - Or a function to extract completion prefix
  - Both `source` and `options` can be given as funcrefs that take the completion
    prefix as the argument and return the final value
- - `sink` or `sink*` are not allowed
+ - `sink` or `sink*` are ignored
 
 
 Reducer example~