Make Row, Column, MaskedColumn, TableColumns be class attributes
Add initial tests of subclassing components
Make the user-subclassable attributes be public (no _)
Refactor pprint funcs into class to allow formatting customization
Make subclass testing more boring
Add subclassed Table into test fixture for broad testing
Add docs on subclassing
Update CHANGES.rst
Add top-level reference to subclassing, fix doc problems
Change t.pprint() to print(t) to fix doctest errs on Mac
Fix RST docs to run correctly and be more clear
Restore new config code lost in rebase
Merge pull request #2287 from taldcroft/subclass-table

Subclass Table components Row, Column, etc