Add rewrite rule to open HN comments w/ iOS apps

[aldur]

Here's my use case: I use hnrss.org as a feed for news.ycombinator.com, which returns entries as follows:

When browsing from iPhone/iPad, clicking the "Comments URL" link opens Safari, instead of HN-specific apps (HACK, for example, or Opener for a more generic solution).

This PR adds two rewrite rules that add a "deep-link" to open the content directly in the app, as follows (the screenshot shows both rewrites applied):

NOTEs:

• In order to make this work, I had to add two additional schemes to the sanitizer's whitelist. Not sure whether this is acceptable. A workaround that doesn't require modifying the whitelist is to to do this: keep a HTTPS scheme and point the link to a web-server which will then redirect the user to the hack:// or opener:// scheme.
• Not sure about the policy for adding rewrite rules; this is useful to me, would be useful to the community as well?

Do you follow the guidelines?

• I have tested my changes
• I read this document: https://miniflux.app/faq.html#pull-request

[fguillot]

I'm okay to merge your pull-request, however it would be nice to have a unit test for the new rewrite rule.

[aldur]

I'm okay to merge your pull-request, however it would be nice to have a unit test for the new rewrite rule.

Makes sense and should have added them earlier. Done :)