Add LEFT_CURLY and RIGHT_CURLY symbols (wikitext, dd43eff)

In order to avoid ambiguity we have to separately tokenize single curly braces and exclude them from the standard PRINTABLE rule. Otherwise the greedy matching behaviour would match a run like "foo{{bar}}baz" as a single PRINTABLE token rather than a PRINTABLE, IMG_START, PRINTABLE, IMG_END, PRINTABLE sequence.

Signed-off-by: Greg Hurrell <greg@hurrell.net>