Add webmention support

[AngeloStavrow]

Is your feature request related to a problem? Please describe.
It would be a better IndieWeb experience to have the theme pull in and display webmentions as suggested in #12.

Describe the solution you'd like
There are services like https://webmention.io that can be used to aggregate webmentions for static sites. However, this will involve adding JavaScript that makes external requests; this should be well documented for users to understand and implemented in such a way as to maintain site download speed, and disabled by default.

Describe alternatives you've considered
None.

Additional context
PR #39, opened by @kisik21, starts us off on this work! 🎉

[xavierroy]

Could you also take alook at https://webmention.app/?

[vikanezrimaya]

@xavierroy webmention.app is used with outgoing webmentions, while webmention.io (and the scope of this issue) is processing inbound webmentions.