Feature request: highlighting support for markdown text

[tridian-tn]

Have you searched existing Github issues to see if someone has already requested this feature?
Yes

Is your feature request related to a problem? Please describe.
When scanning through a page of text in my previous scratchpad s/w (Zim), highlighting support was brilliant for drawing the eye to stuff I needed to keep my attention on from day to day. Heynote supports bolding of text in markdown mode, which is good, but highlighting would be much better for my use case

Describe the solution you'd like
Support for the ==text to highlight== syntax, which is supported by a few markdown editors.

Describe alternatives you've considered
Using bold

Additional context
Add any other context or screenshots about the feature request here.