Make unite.redraw_hold_candidates configurable

### Problem

I use unite.vim to choose a file to edit from all files in one repository.
But some projects I contribute to have 30000+ files.
In such a case, filtering does not happen and I can't select a file to edit.

I tried async sources too, but there are some files which can't be matched.
I want to search a file from all the files.

* Environment information
 * OS: Mac OSX 10.9.3
 * Vim: 7.4.253

### Solution
I realised that its limitation is the same as `unite.redraw_hold_candidates`.
So I want `unite.redraw_hold_candidates` to be configurable.