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
This is generator function for log_appender, for alternatives, see eg appender_console, appender_file, appender_tee, appender_slack