Implement JSON Feed output

[notpushkin]

This currently almost passes JSON Feed validator. The few things it complains about are null values that should instead be removed from the feed. Apart from that and some code review and more thorough testing I think we're good to go!

Closes #165

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d7b5c81) 83.96% compared to head (e1468a6) 84.17%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
+ Coverage   83.96%   84.17%   +0.20%     
==========================================
  Files          10       10              
  Lines         524      556      +32     
  Branches      108      118      +10     
==========================================
+ Hits          440      468      +28     
- Misses         53       56       +3     
- Partials       31       32       +1     

Files	Coverage Δ
mkdocs_rss_plugin/__about__.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/config.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/constants.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/util.py	74.19% <100.00%> (+1.42%)	⬆️
mkdocs_rss_plugin/plugin.py	92.48% <90.90%> (-2.44%)	⬇️

[Guts]

Thanks for your work, it's really cool!

Last steps before complete review and merge:

• add unit tests including compliance tests and some special cases (special chars, etc.)
• complete documentation

[Guts]

@notpushkin any chance to see you fit the missing points in the next days #177 (comment)?

Would you accept that I push to your PR?

[notpushkin]

@Guts sorry for the wait! If you're up to it, please do finish it up for me – my mind is completely somewhere else right now, so I can't say for sure when I'll be able to do it :-( Again extremely sorry for this half-assed PR, but I hope it helps somebody!

[Guts]

Hi @notpushkin,

Better late than never, I took the time to achieve your feature.
It still lacks of compliance validation but I think we can move forward and improve it later.

[notpushkin]

Thank you! Sorry for the radio silence, I've had a lot of things going on this year, way over what I can usually handle :')

Hope this feature helps other users and somebody will be motivated enough to make it fully compliant! It still should be pretty useful in the meantime.

[Guts]

No worries! It was already great to come to this PR. Thanks again.

I'll make a release in a few days.