Improve some features and fix some issues.

Hi @d11wtq 

I like your work (which is very helpful and tiny for me) so I made some changes:

* Add grizzl group.
* Support face customization and fix deprecated faces.
   <https://github.com/d11wtq/grizzl/issues/18>
* Support scrollable result display in minibuffer and truncate display.
   <https://github.com/d11wtq/grizzl/issues/10>
   <https://github.com/d11wtq/grizzl/issues/13>
   <https://github.com/d11wtq/grizzl/issues/14>
   <https://github.com/d11wtq/grizzl/issues/15>
* Support `grizzl-read-update-hook' to info hooks the current result.
* Slightly improve performance by using idle timer to do searching job instead
  of do it in post-command.

TODO:
-----
* Highlight matched characters.
  <https://github.com/d11wtq/grizzl/issues/5>