Add "set editing" command

[rocky]

This is a gdb command but the trepanning debugger don't support mixed emacs/vi commands, but instead basically use GNU Readline undeneath so you set which variant you want.

Here is the help from bashdb:

set editing [on|off|emacs|vi] 

Readline editing of command lines 

See also: 

show editing 

@boretom You up for adding this command?

(In going over documenting remake I came across this.)