Support `<!--` comment syntax

This partially fixes https://code.google.com/p/esprima/issues/detail?id=451.

(After this patch, only the `-->` syntax is left, but that one is a bit trickier (as it [only takes effect when it’s the first token on the line, optionally preceeded by only whitespace or multiline comments](http://javascript.spec.whatwg.org/#comment-syntax)).)