feature(option): allow customize output filenames

[Guts]

Add option feeds_filenames to customize every feed filenames generated by the plugin:

plugins:
  - rss:
      feeds_filenames:
        json_created: feed.json
        json_updated: feed-updated.json
        rss_created: rss.xml
        rss_updated: rss-updated.xml

Default:

• JSON feed for created items: feed_json_created.json
• JSON feed for updated items: feed_json_updated.json
• RSS feed for created items: feed_rss_created.json
• RSS feed for updated items: feed_rss_updated.json

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.65%. Comparing base (c5c3759) to head (222d3f0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   84.60%   84.65%   +0.05%     
==========================================
  Files          10       10              
  Lines         552      554       +2     
  Branches      117      117              
==========================================
+ Hits          467      469       +2     
  Misses         54       54              
  Partials       31       31              

Flag	Coverage Δ
unittests	84.47% <100.00%> (+0.05%)	⬆️

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Files	Coverage Δ
mkdocs_rss_plugin/config.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/constants.py	100.00% <ø> (ø)
mkdocs_rss_plugin/models.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/plugin.py	92.53% <100.00%> (ø)