All changes:

* Added new option, --fetches/-f, to configure how many times each mention should be fetched before the tool closes down. Defaults to 1.
* Added new option, --sync/-s, to provide alternative to --fetches where the tool keeps running until all ping requests has recieved responses.
* Changed how port number is set, now done through new option --port/-p rather than through an enviromental variable.
* Ensured that all instances of the placeholder URL is replaced
* Added WebMention example from notizBlog.org, thanks Matthias!