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