Abstract and Subtitle should be rendered in ODT

Dear all,

I am just about to explore better use of open standards, namely ODT, for my public sector employer and just noticed that both subtitle and abstract is only rendered for DOCX, but not ODT.

This problem has been raised previously in #1451.

The issue #3104 seems to indicate difficulties to support the rendering with the specific paragraph style abstract and with multiple paragraphs. Our use case would be executive summaries, which are often a page long and require separate paragraphs. So I hope that ODT support can be be as good as DOCX support. 😄

Thank you for this great piece of software. It helps us already to produce HTML and PDF output from the same MD file (example).