Updated docs/readline

  * readline.org: added two new keybinds: 'C-x c' and 'C-x C', first
    simply inserts '$(command -v )' placing point before ')', while
    second takes a word and put it into that command;
  * (TOP)/readline/inputrc: updated (tangled from Org document).

Signed-off-by: Vladimir Lomov <lomov.vl@gmail.com>