Add support for purging tasks

[djmitche]

..meaning actually removing them from the task DB. This already occurs when tasks expire (#341), but sometimes (GothenburgBitFactory/taskwarrior#3399) users want to purge a task without waiting for expiration.

It's OK for this to break the sync model a little bit, as long as that's clear in the documentation. #26 has some discussion of this.

[djmitche]

This is referring to purging specific tasks before they expire -- not the existing Replica::expire_tasks support.

[djmitche]

@QuantumBJump would this be an interesting task for you to work on?

[QuantumBJump]

I'll have a looksee, can't promise anything so don't assign yet, but I'll get back to you :)

[djmitche]

#405 should do for this repo. @QuantumBJump I forgot there's already a method for this -- I just had to make it public!

Once that's done, I think we can release 0.6.0 and then implement this (and the server URL) in Taskwarrior.