Allow for "mark" extension

[cagix]

Pandoc supports highlighting text using ==: wuppie ==fluppie== foo bar will be transform into wuppie [fluppie]{.mark} foo bar, when activation the non-default extension mark (like -f markdown+mark).

Let's use this and translate if needed (e.g. into []{.alert} for Hugo or ** ... ** for GFM).

see https://pandoc.org/MANUAL.html#extension-mark

see #206