introduce readibility mode (or extend existing mechanisms)

[humenda]

Alternate texts of formulas are read by people not able to see the rendered images. There are many reasons: no images enabled, bandwidth restrictions or a visual impairment. In either case, reading a LaTeX formula can help to understand the context. For rendering purposes, a lot of formulas contain instructions which make the resulting output nicer to read, but harder to read in the source. Therefore it would be good to implement a replacement strategy for the alternate texts of the images. This could be for instance:

• replace all occurrences of \varepsilon (and friends) with \epsilon
• replace all \dots through ...
• strip formatting instructions like \left or \big