Bind M-C-? to `rl_backward_kill_word`

Makes Option-Delete (what Windows / Linux users would likely call
"Alt-Backspace") work on OS X. Restores parity with GNU Readline.

lib/rbreadline.rb

@@ -466,6 +466,7 @@ module RbReadline
       "\ey" => :rl_yank_pop  ,
       "\e~" => :rl_tilde_expand  ,
       "\377" => :rl_backward_kill_word  ,
+      "\e\x7F" => :rl_backward_kill_word,
       "\C-x\C-g" => :rl_abort  ,
       "\C-x\C-r" => :rl_re_read_init_file  ,