What’s new in unreleased Matplotlib?

Please place new portions of whats_new.rst in the next_whats_new directory.

When adding an entry please look at the currently existing files to see if you can extend any of them. If you create a file, name it something like cool_new_feature.rst if you have added a brand new feature or something like updated_feature.rst for extensions of existing features. Include contents of the form:

Section Title for Feature
-------------------------

A bunch of text about how awesome the new feature is and examples of how
to use it.

A sub-section
~~~~~~~~~~~~~