Add support to extract underline-ish title (setext-style headings)

The util function [`get_markdown_title`](https://github.com/mkdocs/mkdocs/blob/master/mkdocs/utils/__init__.py#L332) supports only ATX-style headers. It should also support:
```markdown
A level-one heading
===================

A level-two heading
-------------------
```