Support for subtitles in templates

[deanmarchiori]

Providing support for subtitles in the templates to improve user options and experience.

Context

In the templates, the title is often the name of the site owner. It may be useful to have a smaller 'subtitle' feild to list post-nomials or job title.

Current result

It seems like the YAML metadata in the templates only support 'title'.

Possible Fix

Im not overly experienced in these templates, but I have modified the pandoc template and skeleton.Rmd for postcards::trestles to allow for this field. Subject to feedback, I would be happy to file a PR?

Example