Implement indentation-sensitive grammar

[This paper](http://michaeldadams.org/papers/layout_parsing/LayoutParsing.pdf) presents a formalism for expressing indentation-sensitive grammars in GLR and LR(k) parsers which they claim easily generalises to various other parsers including LALR.

Would it be possible to extend the production syntax accepted by ParserGenerator to accept this and DTRT? I'd prefer not to insert a hack between the lexing and parsing stages of my compiler to convert whitespace into INDENT/DEDENT tokens.