add readline keybindings

[aeosynth]

elvish adds some initial friction by not supporting common readline keybindings, eg ^H for backspace, ^A/^E for home/end, etc.

for a list of keybindings, see:

https://en.wikipedia.org/wiki/GNU_Readline#Emacs_keyboard_shortcuts
https://cnswww.cns.cwru.edu/php/chet/readline/readline.html#SEC13

[xiaq]

Add use embedded:readline-binding to your rc file to use.

[scateu]

burst into tears.

[aeosynth]

nice. i'd prefer it default, as this is mainly for initial friction, but i'll close.

[orivej]

To save some time for those looking for Unbound: Ctrl-H, the current expression is use readline-binding.