FYI: add customisable titles in more reports' HTML output

[Simon Michael]

Dear all, and particularly Henning: I meant to post this as a pull request, but fat-fingered it and landed it in main instead. So here's what the pull request would have said - 

Previously only the compound balance reports (bs/bse/cf/is) had a title in their html output.

Now balance (single-period, multi-period and --budget), register, aregister and holdings also show a title in html output (the same one as in text output, customisable by --title=TEXT or --title= in the usual way).

And titles in html output are now rendered before the table not inside it; with a report-title class for easier styling; and as a h3 element not a h2 element, to fit better in typical documents.

Note the compound reports (cf/bs/bse/is) have had a h2 title for a long time, and it's now h3, so anyone who was styling those report titles will need to update their CSS.

(I know there are still inconsistencies with title support across reports and output formats, and some overlap between --title and --headings in aregister; I'm not concerned about those for now.)