Add parameter to `unfollow` to delete entries from source

[martymcguire]

Spec section on unfollowing (https://indieweb.org/Microsub-spec#Unfollowing) says:

The Microsub server may remove all of this feed's items from the channel, or may leave them in place, at its discretion. If you are used to treating these channels as an IRC or Slack timeline, it would be more appropriate to leave the old items in the channel, just stop delivering new ones. However if you are more used to treating these channels as a Twitter or Facebook feed, then you may want the server to remove them from the channel.

TODO: Should there be another parameter for the client to specify whether to remove previous entries or leave them?

By way of example, Aperture's implementation of unfollow removes the source without removing its entries. But the remove_source method it call supports a flag to delete the entries.

I'm not sure what this parameter should actually be. Thumbing through the spec I see mentions of actions like channels and timeline using a method parameter. Examples include:

• action=channels supports method values of order and delete
  • delete is used to delete the whole channel
• action=timeline supports method values of mark_read, mark_unread, and remove
  • remove is used to remove one or more specified entries from a channel's timeline.

Given these examples, I propose that an unfollow request could have an extra method parameter with a value remove to both unfollow the source and remove the associated entries.