Add support for FB messenger /silent

[0xdevalias]

When using the FB messenger iOS app I can prefix my message with /silent to have it not trigger notifications/etc on the receiving party's side.

It would be awesome if this functionality was available via this bridge.

I'm not sure if there is any more identifying info at the protocol level, but this is how it comes through on matrix currently:

{
  "room_id": "!REDACTED:beeper.local",
  "type": "m.room.message",
  "content": {
    "msgtype": "m.text",
    "body": "/silent foo bar baz",
    "format": "org.matrix.custom.html",
    "formatted_body": "/silent foo bar baz",
    "_isEmojiBody": null
  }
}