fix(sorting): Fixes the return type being ordered by selector, not position in document

[zakhenry]

This changes the selection so the matches are returned in their natural order from the document, not the order that the selectors were listed.

Now it is possible to make multi level menus.
Example with handlebars:
Metalsmith config:

Metalsmith('test/fixture')
      .use(markdown())
      .use(headings({ selectors: ['h2', 'h3']}))
      .build();

Template:

<ul class="section table-of-contents">
      {{#each headings }}
          <li><a href="#{{ id }}">{{{ifEqual tag 'h3' '&emsp;' }}}# {{ text }}</a></li>
      {{/each}}   
</ul>

Outputs:

# Getting started
# Adding pages
 # Collections
 # Templates & Partials
 # Styles & Assets
 # Variables
# TypeDoc
# Deployment
 # Config
 # Travis CI
# Plugins

Closes #4

[oupala]

Great news. I'm looking forward to see this pull request merged!

[zakhenry]

@oupala if you want to use the feature now - install @xiphiaz/metalsmith-headings - it's just my unmerged fork

[mentos1386]

Would be nice to see this pull request merged.

[oupala]

Thanks a lot @zakhenry !

I'm using your fork as the pull request has not been merged until now.

[oupala]

Very good news, thanks a lot!