Beginnings of subflow support and a bunch of crazy reworking

    The main complication of subflows is that the InlineTag chain
    now supports mixed content.  This is an ocean-boiler.
Subflows in <bpt> working.

    - Subflow in <ept> are implemented but untested.
    - Other tags still to go.
    - Deep nesting untested
    - toXML() is messed up and needs to be fixed.
Add README
Test subflows in ept
Subflow support in <ph>
Subflow support for <it> tags
Remove flawed toXML() stuff; will try again later

    The upshot is that simple text/code content is a little more
    difficult to deal with.