Support setting a base URL for data-include

[sidvishnoi]

When building specs with data-include, it would be helpful be able to dynamically set a dataIncludeBase URL in respecConfig. This could help:

• add data-include support with pr-preview (See https://github.com/w3c/respec/issues/4480, Support for relative URLs tobie/pr-preview#144),
• give us ability to use different versions of data-include at same path (say using different commit as preview).
• Also in docs at respec.org, we set the absolute data-include URL as a preProcess script to avoid duplication as well as being able to edit locally (using a localhost static server in cloned wiki).