Allow multiple base_urls for webmentions' sake

[jmacdotorg]

If a Plerd-based blog adheres the now-common practice of forwarding http:// requests to https://, then it won't recognize a well-intentioned webmention sent to the http://-based URL. And this more generally holds true for all situations where a blog quietly redirects all requests to other URLs derivable from the requested ones.

One way to fix this would be to support multiple base_urls in the config -- or one base_url but several recognized aliases. Plerd would then (internally) file its post objects under multiple URLs, making it more flexible about what webmentions it accepts.

[jmacdotorg]

A day later, this solution to a real problem feels too complicated.

A simpler fix: Allow the user to define a filter function which all webmention target URLs would pass through. The function can do anything it wishes, and outputs another URL at the end. (Perhaps transforming the input URL, perhaps not.) Plerd would then use that filtered output URL for the lookup, without the need for any further changes to business logic or internal structure.