- replace mf2j with mf2clj
- remove utils.clj completely
- refactored processor, use Chain of Responsibilities pattern and pass
around maps to reuse data (so we don't have to make a separate HTTP
request just to parse microformats, etc.)
- test: add more test cases to verifier test
- test: add basic test cases to all processing steps
- add more logging to each processing steps. good for debugging.

- processor will call hook functions with webmention data
- refactor webhook.clj massively
- add dynamic parameter substitution for urls, e.g. {source} {target}
{event}......
- add config option :webhook-request, a function with parameters [hook,
data] that returns a map (resembling {:headers ... :body ...}).
webhook.clj uses this function to generate payload.
- allow map besids URL string in config option :webhook. if it's a URL,
sends webhook request to it with payload generated by :webhook-request;
if it's a map, it should be like {:url ... :request-fn ...}, which
specifies a payload generator.
- add request-fn preset :ntfy

processor: tidy up log msg and level; improve code style

- all messages start with a lower-case letter
- all webmention-related logging contains first 8-digit of its id
- use WARN level when something is deleted, not INFO
- use WARN level when exceptional cases happen, not ERROR
- get rid of emojis in log messages

utils: digest-short function for logging

verifier: tidy up log messages; improve code style

- use `cond` instead of nested `if`s
- same message format as processor's logs

webhook: tidy up log messages; send webhooks robustly

- use same logging formats as processor and verifier
- handle error, 4xx, 5xx status and log them
- move logging inside webhook module, away from core.clj

Content-Type checks are somehow written in process-webmention function,
when it should be in verify-mention function, which causes weepinbell
not deleting Webmention when one of target and source is not found (or
any other HTTP status code that is not 2**)

you can't test it if it's private

process-webmention used to start a go-loop directly.
this commit separates go-loop from processing logic,
core.clj now calls to processor/start, the wrapper
function.