Support recenter on C-l

Hi there,

OK, this merge request thing works differently than I imagined.  I guess I didn't need to close the previous merge request, and this one will end up being a merge of two commits.

This adds some functionality that's missing from phisearch as compared to isearch.  First is C-l for recenter, second is C-w for yank.  The former, I believe, is uncontroversial.

The latter is a bit problematic, as the user might want to use C-w to edit the search buffer itself, so we first check whether there's an active region.  Both of these commands I have in my muscle memory and feel should be in phisearch.

Thanks,
PM