Feature: "filter_shortcodes" integration

[agwells]

As @danmarsden mentioned here: https://github.com/catalyst/moodle-tool_trigger/issues/6#issuecomment-389317576

FYI - @FMCorz (ex Moodle HQ dev) has recently developed a shortcodes filter - I haven't had a chance to look at it closely but I wonder if we should be using that format - It looks like there's also potential for adding a new atto plugin for this listing all the available shortcodes which could potentially make it easier for admins to set the triggers up.

https://github.com/branchup/moodle-filter_shortcodes