YAML metadata title using causes error in EPUP generation

[ricardodelagarza]

When use YAML format metadata in .MD document as suggested:

---
title:
- type: main
  text: My Book
- type: subtitle
  text: An investigation of metadata

Bring the following error:

[WARNING] This document format requires a nonempty <title> element.
  Please specify either 'title' or 'pagetitle' in the metadata.

In previous versions it worked fine. The current work around is use simple title like:

---
title:  My Book 

Although you may not use the documented alternatives:
Title
Either a string value, or an object with fields file-as and type, or a list of such objects. Valid values for type are main, subtitle, short, collection, edition, extended.

[jgm]

Yes, I think we need to change the EPUB writer so that, if a structured value is given for 'title', it constructs the title-meta value appropriately.