Present is pretty simple-minded. This feature seems outside its requirements.

@robpike actually I found a simple way for doing this, with .html function.

If anyone want mathematical symbols, here's tutorial:

1. Save <script...></script> that includes your favourite JS syntaxer as math.html
2. Add .html math.html to any of slides (best I think is to add to last one)
3. It will magicaly work, you can test by writing$$x_i = i\prime$$ in any slide and seeing it rendered.