Add an "IETF mode" capability to use a different CSS and layout - and perhaps other SDO formats

It would be great to be able to trigger a different layout from the spec based on an "sdo_format" configuration setting, and perhaps a set of underlying configuration values.

Specific to IETF, there is not a need (IMHO) to use the old fully justified, courier font layout, but things like the header format, the placement of the table of contents and so on to adhere to the IETF style would be nice.

For now, we are using SpecUp in the current style, with a plan to mechanically transform the document if and when an IETF version is needed. The features of SpecUp make it way too useful vs. tools specific to other SDO formats that we've been able to find.