Tab indentation support

[toastal]

Tabs are not allowed in Hamlet indentation

What is the technical limitation for this? Both Haskell & HTML support tab indentation with tabs also being the more accessible indentation option over spaces. I see old discussions about confusion with mixed indentation in the parsing rules, but banning tabs seems like the wrong call versus requiring users stick to one or the other.

#285 wishes for pasting HTML, but pasted HTML can & will have tab indentation.