Consider non-vacat space for indentation

[hcayless]

@jcowey mentions the need for various levels of indention, and I wonder whether instead of using <lb rend="indent"/> we could use <space>. If you could do something like:

<lb n="2"/><space extent="5" unit="character" type="indent"/>...

then instead of rendering it with "vac. 5" or something like that, we'd just give it 5 em's worth of blank space.

I'm contemplating whether this could work for "outdent" as well, but that's possibly mad. Note that this would have implications for papyri/xsugar#29 as well.

[jcowey]

italicize and get rid of question mark and make slightly smaller.