Add an option for multi-tier regex matching

Add an option for multi-tier regex matching

* ivy.el (ivy--regex-plus): New defun. This is an example for the
  multi-tier interface.
(ivy--filter): Update.

To use it, add e.g.:

(setq ivy-re-builders-alist
      '((t . ivy--regex-plus)))

Example using boost_1_58_0 and `find-file-in-project`:

* "utility" - 234 matches
* "utility hpp" - 139 matches
* "utility !hpp" - 95 matches

Fixes #62