Style display format
Closes https://github.com/pydata/pandas/issues/11692
Closes https://github.com/pydata/pandas/issues/12134
Closes https://github.com/pydata/pandas/issues/12125

This adds a `.format` method to Styler for formatting the display value
(the actual text) of each scalar value.

In the processes of cleaning up the template, I close #12134 (spurious 0)
and #12125 (KeyError from using `iloc` improperly)
