Add Ctrl+h and Ctrl+m keybindings

[un-def]

As an ex-dmenu/rofi user, I really miss these two terminal/shell-style keybindings:

• Ctrl+h → Delete the preceding character (0x08, BACKSPACE)
• Ctrl+m → Confirm (0x0D, CARRIAGE RETURN)

There is another commonly used keybinding for Confirm — Ctrl+j (0x0A, LINE FEED) — but it is already mapped to Move Forward (anyway, I personally never use Ctrl-j).