[osis-core] <hi> types

[Chris Little]

I'm working on an XML version of das Nibelungenlied, and realized that I 
should be retaining some indication of illuminated letters.  Then it 
occurred to me that we should have some way of handling this in OSIS 
too, if we intend to support manuscripts.  Then it occurred to me that 
<hi> would do a pretty good job of filling this role.

Since we don't have any types defined for the <hi> type, I would like to 
suggest that we add some to cover:

silly presentation stuff like boldface, italic, underline, small caps, ...
and real stuff like illuminated letters, maybe drop caps, perhaps those 
lengthened Hebrew letters, ...

--Chris