Android: Support html attributes in body notifications

[ollm]

Feature request

I'm migrating from phonegap-plugin-push and something I miss in this plugin is being able to put certain words in bold format in notifications body.

https://github.com/phonegap/phonegap-plugin-push/blob/master/src/android/com/adobe/phonegap/push/FCMService.java
https://stackoverflow.com/questions/45593710/how-to-bold-text-in-a-notification

Example:

{
  "name": "my_notification",
  "notification": {
    "body": "Tomorrow it will be <b>28º</b>",
    "title": "Weather in Barcelona"
  },
  "data": {
    "notification_body": "Tomorrow it will be <b>28º</b>",
    "notification_title": "Weather in Barcelona"
  }
}

[dpa99c]

Resolved by #668 which will be included in next release.