Change the default of group_name_formatter to str.title

[hamdanal]

Long group names formatted in upper-case look weird and are harder to read than lower-case or title-case. The default of group_name_formatter must be "friendlier" with both long and short group names. str.title looks like a more convenient default.

• str.upper:
  

• str.title:
  

Changing this could break tests that depend on the default format but it is better done now than after a 1.0 release.