Allow reporting delay to be applied to different date events

[joshwlambert]

Currently the $date_reporting column is the date of symptom onset plus any user-specified reporting delay distribution. Given in real outbreak scenarios that the reporting date can be after symptom onset, hospital admission or outcome it would be useful to allow the user to adjust what the reporting delay is added to.

This can be added as a option in config, e.g. config$reporting_trigger = "onset", but could alternatively accept "admission" or "outcome".

[joshwlambert]

Pushing this feature to v0.6.0 because the package is now ready for a new release to get the new functionality (incl. messy_linelist() and truncate_linelist()) and bug fixes on CRAN and available to all users.