Make server capability warnings optional

[PhrozenByte]

notify-send.sh shouldn't print server capability warnings (like the device not supporting hyperlinks in the notification body) by default, but rather hide them behind some flag (like --verbose or the LOGGING env variable):

$ src/notify-send.sh Test
Warning: The notification service on your device doesn't support
         hyperlinks in it's body text. So they will be filtered out.
Warning: The notification service on your device doesn't support
         images in it's body text. So they will be filtered out.

Moving this from #1 (comment) and #1 (comment) here since it's a different issue.