"Trailing punctuation in heading" confuses emoji codes with punctuation

[mlibre]

In header like:

# Header :heartpulse:

It can't detect where ":" is for emoji code or it is an unnecessary ":"

[DavidAnson]

Emoji are not part of the CommonMark specification, but I'll look into this.

[DavidAnson]

See #653 for markdown-it plug-in.