Support HTML5 element id

[Dafrok]

In HTML5, you can take any unicode and use them as values for id
attributes. It’s important for non-english languages. For example:

# 蛤蛤

will be complied by this lib into:

<h1 id="-">蛤蛤</h1>

however, the majority of us want it to be:

<h1 id="蛤蛤">蛤蛤</h1>

[BOT-Man-JL]

I suppose overriding renderer.heading method and using Unicode filter will help (walk around)

raw.toLowerCase().replace(/[^\w\u4E00-\u9FFF]+/g, '-');

Ref: https://en.wikipedia.org/wiki/CJK_Unified_Ideographs

[joshbruce]

Closing due to merge conflicts and not necessarily the focus at this time - flagging with #981 for future reference.