• Small tweaks and dependency updates

• Support Content Negotiation

1. Refactored to match the configuration of feeds built into WordPress
2. Bumped PHP Version requirement to PHP5.6 to match WordPress 5.3
3. Bumped minimum WordPress version to 5.2 as this allows for the version of get_content that includes a $post parameter
4. Fixed incorrect PHPCS configuration
5. Enabled JSON Pretty Print by default as originally disabled due a PHP5.4 requirement
6. Changed Post Item Generation Class to use WordPress functions instead of directly accessing the data where applicable
7. Adjusted jf2 feed to comply with jf2feed spec (https://jf2.spec.indieweb.org/#jf2feed)

• Fixed JSON output
• Fixed "flush rewrite rules" again

• Fixed "flush rewrite rules"
• Added filter to extend the mf2/jf2 data

• Complete re-write to match the latest ideas of rel-alternate: indieweb/wordpress-uf2#38

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