Support backward search with 3-1 chord, add flag to swap default search direction

[BenWibking]

It would be very useful to be able to easily do a backward search as easily as a right-click forward search. A mouse chord could be used to enable this.

Also it would be nice to have an option to switch the default search direction to backward.

For feasibility, see e.g.
https://groups.google.com/forum/#!topic/comp.os.plan9/Q6R9iuu0lE8
https://bitbucket.org/mpl/acme-with-easy-backwards-search/src/bc767fc346e75e1cc949bd3c09c7e0253493ba08/patch?at=default&fileviewer=file-view-default