Add readline-like reverse intelligent search on ctrlR key press.

Use ctrlR to enter search mode, and then use ctrlR and ctrlS to navigate
backward and forward.
Search: Implement Ctrl-G (Cancel)

Ctrl-G restores the original line and cancels the search.
Document Ctrl-R (Reverse Search)
Merge remote-tracking branch 'upstream/master'

Conflicts:
	line.go
Changes required after merge.