Note: this also adds special syntax to templates allowing to specify
custom prefix for templates;

It also strips any undefined variables from templates.

This just resets all counters that have new element prefix in "scope".

Closes #190

- tests are mostly broken
- subfigures are broken
- chapters are broken

Notable changes:

-   Unrecognized references are converted to citations
-   Chapters should be possible via scoping rules, but it's not as easy
    as it should be
-   Subfigures should be possible via scoping rules to a limited extent,
    but there's a lot more to do for it to be feasible.

TODO: perhaps allow overriding via an attribute

Main thing: don't error out on undefined prefixes

-   Attribute overrides: some reference properties can be overridden by
    attributes
-   metadata tweaks: use nbsp between item title and item number in
    captions.
-   more generic templates: use typeclass function makeTemplate for
    all template types
-   removed LatexPrefixes: wasn't really used; the initial motivation
    for it didn't pan out

-   autoEqnLabels now specifies prefix for auto labels
-   added autoFigLabels
-   last para in reference-able div is treated as caption if
    it starts with `: `
-   prefixes of different nesting level can be fully customized using
    new prefix definition `sub` property
-   removed awkward dependency on nesting level from prefix properties
    (still possible via templates)
-   new prefix settings `subcaption` and `subcaptionGrid`

This is somewhat limited, since it only shows captions collected above the place where it's inserted. Only collects items which are immediate children of the current scope.

Currently a placeholder for collected captions is empty span `[]{}`. Might be changed in the future.

…sewhere

This commit removes most LaTeX-specific code, and just treats LaTeX as any other output format. This should (mostly) work for the purpose of PDF output, but the resulting LaTeX code wouldn't be much different from a plaintext file. Anyway, reimplementing pandoc-crossref algorithms in LaTeX might be a fun challenge, but it will take an unreasonable amount of time. LaTeX gurus are welcome to help out.

…streamline templates;