Generalised more in Parsing.hs to enable the use of custom state

If I remember correctly there was a reverted commit a while back which was moving away from having the same state for each reader. This commit adds a new type class, HasQuoteContext, which moves further in this direction.

1. New type class, HasQuoteContext.
2. More functions are properly generalised
3. HTML reader now uses a custom state in preparation for the epub changes. This exact specification is likely to change as I continue generalising Parsing.hs and working on the EPUB reader.