Add key combinations for ctrl-delete and shift-delete (#3284)

Currently there is not option to bind ctrl-delete and shift-delete. As
suggested by issue #3240, shift-delete could be used to bind "delete
entry from history" as it is a common way to do so in other
applications, e.g. browsers.

This, however, does only implement to use the key combination itself and
does not assign a default action to any of them. This does enable to
call one's all predefined actions. With the exec action this can
expanded like the issue #3240 suggested.
If desirable, the key combinations could later get a default behavior.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

man/man1/fzf.1

@@ -873,6 +873,8 @@ e.g.
 .br
 \fIctrl-space\fR
 .br
+\fIctrl-delete\fR
+.br
 \fIctrl-\\\fR
 .br
 \fIctrl-]\fR
@@ -941,6 +943,8 @@ e.g.
 .br
 \fIshift-right\fR
 .br
+\fIshift-delete\fR
+.br
 \fIalt-shift-up\fR
 .br
 \fIalt-shift-down\fR