revise doc/design_narratives on rels handling
switch off pretty print on XML output
add WMF and BMP image types
fix case insensitive _ContentTypesItem lookup

ISO/IEC 29500-2, section 10.1.2.3 and 10.1.2.4 specifies that lookup of
content type from partname (override) or extension (default) should be
case insensitive. This patch adds that behavior.

Incidentals:
* create separate test class fixture for __getitem__ method of
  _ContentTypesItem.
* change __defaults and __overrides local variables to single leading
  underscore
* initialize _defaults and _overrides to empty dict instead of None
* remove guard check for _defaults or _overrides is None on __getitem__
* whitespace adjustments
doc: document font properties analysis
spec: fix mis-spelling of prst 'foldedCorner'
shapes: _Font.bold = None clears bold attribute

The 'b' attribute on rPr is actually a tri-state setting. It can be
"true/1", "false/0", or not present. When not present, the bold setting
is inherited. Adding None as a valid setting adds the option of clearing
the value to the prior options of setting it either True or False.
api: rename dunderscore locals to single underscore

Also added noqa flag in test/context.py to avoid flake8 complaint.
deploy: update mailing list in README.rst
spec: fix iss39 add x-fontdata part type