Update Google Analytics, add multi-author support, and fix menu-highlighting.

[ajaquith]

Hi! I have been enjoying your Mediumish port and have proposed some changes. These include:

• Added support for multiple authors (the pages for which are stored in an /authors directory). At present, information for only one author is supported, and all information is stored as site parameters.
• Fixed an issue where navbars were not highlighting consistently based on position. In the fix, it will highlight the navbar menu in both cases; if you are on the navbar page or in a page that is inside a navbar item.
• Added support for a custom date format in post dates; this appears both in postbox and in the posts themselves.
• Added support for a simple favicon template, which can be overridden by people who care about those sorts of things.
• Updated documentation to reflect the foregoing, plus: loading Mediumish as a Hugo module (preferred!), using GA4 analytics, and the mainstream config file names (they are now hugo.toml and hugo.yaml).
• Tweaked the arrangement of the post meta information to more closely align with how Medium does it these days (name on top line, then on the next line: clock, reading time, and post date).
• Added some minimal table styling, and tweaked the footnote and caption styles so that they are decent.

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