Send log messages to Pushbullet
appender_pushbullet(...)parameters passed to pbPost, such as recipients or
apikey, although it's probably much better to set all these
in the ~/.rpushbullet.json as per package docs at
http://dirk.eddelbuettel.com/code/rpushbullet.html
Other log_appenders:
appender_async(),
appender_console(),
appender_file(),
appender_kinesis(),
appender_slack(),
appender_stdout(),
appender_syslog(),
appender_tee(),
appender_telegram()