ENH: display_format for style

Closes pandas-dev#11692
Closes pandas-dev#12134
Closes pandas-dev#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 pandas-dev#12134 (spurious 0)
and pandas-dev#12125 (KeyError from using iloc improperly)

cherry pick test from pandas-dev#12126

only allow str formatting for now

fix tests for new spec

formatter callable

update notebook

Author: TomAugspurger