Allow pinging from an old URL

[fluffy-critter]

Currently, pinging an entry by URL will only send the pings from the current canonical URL. Sometimes it's necessary to manually send pings from an old or non-canonical URL; for example, forcing a receiving website to update the canonical URL on an outdated ping (when the interim update failed for whatever reason, or the user hasn't been using the caching mechanism).

So, there should be a command-line option that makes process_entry_mentions send the pings from url instead of entry.url.

[fluffy-critter]

Possibly two options: one for "use the original URL" and one for "use this specific URL," the latter of which being necessary for situations where the URL in question doesn't provide the mention targets at all (e.g. the entry was deleted outright).