Make HTMLEncoding optional

[bruno-brant]

Currently Liquid enforces HTML-encoding of all properties (and in many places). This is a very particular behavior that, I'm sure, is not necessary on most cases.

If there's still use for it, it should at least sit behind a flag.

Also, the encoder should be cached to avoid taking a performance hit on every.... property... of every object!

[bruno-brant]

Marked it as priority because we already saw the encoding conflicting with other functionality.