thoughts on how to handle table cell formatting

+ I think "double parsing" is required, so **disallow table cell formatting if we cannot tolerate double-parsing**

+ double parsing **can** be done inside Pod.nqp with a sub instead of calling the parser

+ the parsing would result in a twine for each cell

+ handling would require renderers to recognize each table cell contains a twine structure rather than a simple string

+ consider making a twine a first-class object

+ document the twine structure (or object) inside the code as well as in the glossary