Add option to show whitespaces

[geroldmeisinger]

Especially for a reference implementation of elastic tabstops I think it helps a lot to understand what's going on with whitespaces enabled.

I use the following symbols:

· space
→ indentation tab
↔ elastic tabstop

(note that in CSS-based renderers if a symbol is missing in the font it gets replaced by a character in the next font.. which makes monospaced fonts effectively not-monospaced)

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