Support recenter on C-l

- Since the edit buffer will typically be small, we can assume that
  the user doesn't really wish to recenter the edit buffer, but the
  target buffer.  This is how isearch behaves as well.
Yank word on C-w

- If a region is active in the search buffer, yank that.
- Otherwise expand search like isearch does.
Merge pull request #6 from pmachata/master

Support recenter on C-l