% Make tags available

This is the difficultmost. Collect YAML meta block *tag* and *tags*
fields (as scalars or lists) and amend to the mainline tag work.

## Info

Tags and labels are used to for many things, such as filtering for
inline. The [tag directive][drtv:tag] is standard usage. Maintenance
is through [tag plugin][pln:tag].

[drtv:tag]: https://ikiwiki.info/ikiwiki/directive/tag/
[pln:tag]: https://ikiwiki.info/ikiwiki/plugins/tag/

## Progression notes

Questions, which is execution order? If parsing with *htmlize* happens
before, that should be a good start.

Possible places for source code: */IkiWiki/Plugin/tag.pm*. After
copying to local dir, and adding some debug code, this is snippet of
output:


```
D: tag:import: Run.
D: pandoc:import: Run.
D: tag:getopt: Run.
D: tag:checkconfig: Run.
D: tag:preprocess_tag: Run for test, test and 0.
D: tag:preprocess_tag: Run for test, test and 0.
D: IkiWiki.pm:htmlize: Run for test, HASH(0x563f41723878),
D: pandoc:htmlize: Run for test
D: pandoc:htmlize: Pandoc command:  pandoc -f markdown -t json --email-obfuscation=none
D: pandoc:htmlize: Pandoc command:  pandoc -f json -thtml5 --email-obfuscation=none--filter=pandoc-citeproc
D: pandoc:htmlize: Return.
D: pandoc:pageactions: Run for test
D: tag:pagetemplate: Run for test, test, page.
D: pandoc:pagetemplate: Run for test and page
```

Thus it seems tag plugin gets all the execution before Pandoc plugin,
for pulling all tag related info. This will be a hard one to break.