2019 replaced by 2020 in line 94.

Co-authored-by: Patrick Marsceill <pmarsceill@users.noreply.github.com>

Co-Authored-By: Patrick Marsceill <pmarsceill@users.noreply.github.com>

Improved search documentation

Added multiple previews, configurable by site.search.previews
Improved search-data.json

Improved documentation for search configurations

- Introduced "ga_tracking_anonymize_ip" parameter to enable/disable Google Analytics IP anonymization (to comply with GDPR).

(cherry picked from commit ce530f36fa0549c78ffe53ea6077f44f6f0b330f) (+1 squashed commit)
Squashed commits:
[69b7718] - enable GA anonymize_ip

(cherry picked from commit f2b67c632af72b61dd634b9a337200781519691e)

This reverts commit 7220197a.

Added documentation for `search_tokenizer_separator`.

Mathjax config options added.
`compress_html` config option changed,
Mathjax script loaded by head when `page.mathjax` is truthy.
Documentation and test page added.
See also issue #198.

(cherry picked from commit dffb2706a158784e2f3091f895a868e373683bc8)

This commit makes the dark mode preview feel a little more interactive
and it also pulls some duplicated code into one file so that people like
me aren't stumped when they are looking at the wrong page!