Add default ctrl+c binding for completion yank.

By default, ctrl+c will yank the selection from the completion menu onto
the clipboard, and ctrl+shift+c will yank it onto the primary selection.

Unfortunately ctrl+y was already taken by rl-yank (which,
counter-intuitively to vim users, will paste the last deleted text).

qutebrowser/config/configdata.yml

@@ -2245,6 +2245,8 @@ bindings.default:
       <Ctrl-Shift-Tab>: completion-item-focus prev-category
       <Ctrl-D>: completion-item-del
       <Shift-Delete>: completion-item-del
+      <Ctrl-C>: completion-item-yank
+      <Ctrl-Shift-C>: completion-item-yank --sel
       <Return>: command-accept
       <Ctrl-B>: rl-backward-char
       <Ctrl-F>: rl-forward-char