Allows h1/h2/h3/h4/h5/h6 tags to preserve their dom id

[fengmk2]

e.g.:

# h1

## h2

### [h3](/already/linky)

#### h4

##### h5

###### h6

# h1 foo bar 中文 no-ascii should be ok_-/

=>

<h1 id="h1">h1</h1>
<h2 id="h2">h2</h2>
<h3 id="h3-%2FFalready%2Flinky"><a href="/already/linky">h3</a></h3>
<h4 id="h4">h4</h4>
<h5 id="h5">h5</h5>
<h6 id="h6">h6</h6>
<h1 id="h1-foo-bar-%E4%B8%AD%E6%96%87-no-ascii-should-be-ok_-%2F">h1 foo bar 中文 no-ascii should be ok_-/</h1>

Should I implementation this with syntax extensions?

[puzrin]

gjtorikian/html-pipeline#111 (comment)

Simple implementation will be unsafe (both via id & name) or not universal (with prefixes), that's the main reason why we not added heading anchors by default yet. That needs discussion.

Right now it's better to implement those locally via md.renderer.rules.header override, if you need those and can't wait. Or, if you decide to publish plugin - write there with "big letters" about possible security problems on unproper use.

[fengmk2]

OK, I try locally first.

[puzrin]

I've structured prelimibary info in #28

Anchors are required - no question. Just don't know what to do by default, to protect users from obvious security holes.

[fengmk2]

mv to #28