unify data_bool

[john-friedman]

xml extension should be part of data bool

document.data on xml -> standard xmltodict or modified (careful due to existing xml2dict)

document.data_tuples -> figure a way to unnest xml
document.text -> text representation of xml
document.markdown -> similar

This is low priority and a bit silly but will simplify code dramatically.

Also makes it easy to do operations such as NLP over text stored within XML files.