#158 Highlighter modes

[Anonymous]

Current highlighter simply mixes (based on alpha values I guess) the highlighter color with the underlying color. It would be nice if there were different modes to add the highlighter color: for example multiply or darken to preserve dark text on light background readability. (see https://en.wikipedia.org/wiki/Blend_modes\).

I'm attaching an image to show the difference between various blend modes.

[Nobody/Anonymous]

Different highlighter blend mode improve readability of highlighted text.