Added Unicode Bullet character (U+2022) as valid list item indicator

There has been a [discussion](http://talk.commonmark.org/t/unicode-character-bullet-u-2022/397) where various reasons have been given why it would be nice if this character would be supported. For me the main reason is the ability to copy-paste (copying lists from, for example, Word or IE adds this character) without the need to manually change the bullets.

The PR includes change to the spec.txt, JS implementation and C implementation.

All tests pass for both JS and C implementation.