patch: grab keyboard input

[SimonTeixidor]

This patch adds a flag, -k, which causes xnotify to grab the keyboard until the user presses the ESC key, which will also close the open notification.

I have these use cases in mind:

• Urgent notifications ("BATTERY LEVEL BELOW 5%!", "MEETING IN 1 MINUTE"), as grabbing the keyboard "forces" you to interact with the notification
• In combination with dmenu, in scripts. For example, and image viewer can be implemented like:

#!/bin/bash
set -o pipefail
while find "$1" -regex ".*\.\(jpg\|png\)" \
        | dmenu \
        | xargs -I {} printf 'IMG:{}\t{}' \
        | ~/xnotify/xnotify -k; do :; done