FIXME: hugolib: markdown_title: add title from markdown

FIXME: hugolib: markdown_title: add title from markdown

Getting title from the page itself, as first encountered header in
markdown file.

Currently it's inefficient as we parsing page twice, the first in some
ealier point when hugo renders markdown, so it should be embedded
somewhere in that place.

But currently i don't care about if a PoC implementation.

Currently it's required to use '.MarkdownTitleInfo.Title' instead of
'.Title', where appropriate.

TODO:
- make it config controllable
- embed it into .Title:
  - fallback to Meta if no markdown title available
  - or do the opposite and use it if no Meta provided

Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>