JATS writer should include <p> or <title> tag in image caption

[arfon]

Problem

Simple Markdown figures seem to fail the PubMed Central validator as they are missing the required tags around the caption:

Pandoc version

pandoc 2.6

Steps to recreate

This URL on Try pandoc

![Figure 1. Citation counts for PLOS Biology articles published in 2010. Scopus citation 
counts plotted as a probability distribution for all 197 *PLOS Biology* research articles 
published in 2010. Data collected May 20, 2013. Median 19 citations; 10% of papers 
have at least 50 citations.](/images/2013-12-11_figure_1.svg)

produces

<fig>
  <caption>Figure 1. Citation counts for PLOS Biology articles published
  in 2010. Scopus citation counts plotted as a probability distribution
  for all 197 <italic>PLOS Biology</italic> research articles published
  in 2010. Data collected May 20, 2013. Median 19 citations; 10% of
  papers have at least 50 citations.</caption>
  <graphic mimetype="image" mime-subtype="svg+xml" xlink:href="/images/2013-12-11_figure_1.svg" xlink:title="" />
</fig>

Desired behaviour

Produce either:

<fig>
  <caption><title>Figure 1. Citation counts for PLOS Biology articles published
  in 2010. Scopus citation counts plotted as a probability distribution
  for all 197 <italic>PLOS Biology</italic> research articles published
  in 2010. Data collected May 20, 2013. Median 19 citations; 10% of
  papers have at least 50 citations.</title></caption>
  <graphic mimetype="image" mime-subtype="svg+xml" xlink:href="/images/2013-12-11_figure_1.svg" xlink:title="" />
</fig>

or

<fig>
  <caption><p>Figure 1. Citation counts for PLOS Biology articles published
  in 2010. Scopus citation counts plotted as a probability distribution
  for all 197 <italic>PLOS Biology</italic> research articles published
  in 2010. Data collected May 20, 2013. Median 19 citations; 10% of
  papers have at least 50 citations.</p></caption>
  <graphic mimetype="image" mime-subtype="svg+xml" xlink:href="/images/2013-12-11_figure_1.svg" xlink:title="" />
</fig>