Add support for videos

[JGilbert-eLife]

eLife publishes videos on the same level as figures. They can have source date and source code. It should be possible to cite them in the text as, e.g. <xref ref-type="video" rid="video1">Video 1</xref>.

<media mimetype="video" mime-subtype="mp4" id="video1" xlink:href="elife-00666-video1.mp4">
                        <object-id pub-id-type="doi">10.7554/eLife.00666.016</object-id>
                        <label>Video 1.</label>
                        <caption>
                            <title>
                                A description of the eLife editorial process.
                            </title>
                            <p><supplementary-material id="video1sdata1">
                                <object-id pub-id-type="doi">10.7554/eLife.00666.037</object-id>
                                <label>Video 1—source data 1.</label>
                                <caption>
                                    <title>Title of the source code.</title>
                                    <p>Legend of the source code.</p>
                                </caption>
                                <media mimetype="application" mime-subtype="xlsx" xlink:href="elife-00666-video1-data1.xlsx"/>
                            </supplementary-material>
                            </p>
                        </caption>
                    </media>

eLife also allows a specific variant of videos, animations, which play continuously online. This labelling is applied to videos with mime-stubtype="gif".

<media mimetype="video" mime-subtype="gif" id="video2" xlink:href="elife-00666-video2.gif"> 
                        <object-id pub-id-type="doi">10.7554/eLife.00666.038</object-id>
                        <label>Animation 1.</label>
                        <caption>
                            <title>A demonstration of how to tag an animated gif file to ensure it is autolooped when on the eLife website.</title>
                        </caption>
                    </media>

[michael]

Thanks for tracking this. Need to think how we treat videos inside a Dar. For pure self-containment they should be included but this can lead to quite heavyweight Dars.