docs(README): list available --style components in Output style section

The Output style section explained how to combine values into --style
without listing what those values are, so the recommended path was
either to read the man page or grep the source. Adds two tables: the
four pre-defined styles (default, full, auto, plain) and the eight
individual components (changes, header, header-filename, header-filesize,
grid, rule, numbers, snip), copying the descriptions from `bat --help`
verbatim. Also calls out the default-enabled set explicitly so the
"by default..." sentence in the existing tip block has a referent.

Closes #3228