Sidebar header extraction improvements

[yyx990803]

Currently with the following titles:

## `some code`

## <foo>

## [Some link](/)

When extracted into the sidebar they are displayed verbatim. We need to improve the title inference / header extraction logic to handle these cases (Emoji was fixed previously in #206 but it was only one of the many possible cases).

[jlooper]

What if one could write something like this:

## `some code` | This is wicked cool code

Then the part after the pipe could be the displayed title? I've been tinkering with side bars recently...

[ulivz]

Cool, some questions:

1. Just removing `, or keep the <code> style?

## `some code`

2. Just unescaping it to <foo>?

 ## <foo>

3. Just removing the link caontainer ?

## [Some link](/)

[ulivz]

Will release it at 0.9.0: #419. 🚀