implement css styles on underline

[opoudjis]

Follow-on from relaton/relaton-render#21

This is not at all urgent: it is a feature introduced for completeness, and I don't anticipate we will encode Taiwanese bibliographies in a hurry.

But there is a requirement for Traditional Chinese titles in bibliographies to be presented with a wavy underline. To realise this, I have introduced a CSS style attribute to underline, with the expectation that the keyword values of text-decoration-style, if present in the style attribute, will be respected: solid, double, dotted, dashed, wavy. So the Taiwanese underlines will be marked up (if they ever occur) as <underline style="wavy">

If this is at all difficult to do in XSL:FO, feel free to ignore.

[Intelligent2013]

In XSL-FO there isn't property for wave underline/border, and in Apache FOP respectively. It's available in AntennaHouse XSL-FO processor only via extension https://www.antennahouse.com/hubfs/xsl-fo-sample/block/axf-border-wave-form-1.pdf?hsLang=en
So let's postpone it.

[opoudjis]

If we ever do encode a Traditional Chinese document...