List items can be enabled, removed based on selections

[mixonic]

Any list items currently under select should be toggleable.

A list item under cursor should remove list formatting when toggled

Content-Kit:

Google Docs:

Several selected sections should get list formatting when toggled, unless they already have it

Content-Kit:

Google Docs:

[bantic]

wip on https://github.com/bustlelabs/mobiledoc-kit/tree/toggle-list-sections-186