Entry point for custom actions module

[scarroll32]

Provide a simple way to extend Typewriter based on selections, applications could be

• pop up styling menu (aka Medium)
• additional context for sections of text (word lookups, audio etc)

[jacwright]

Edit: updates for API changes

This is a documentation task. To do this you would:

• listen to the change event
• check if the selection is not null and not collapsed (isCollapsed = selection[0] === selection[1])
• get the selected bounding rect rect = editor.getBounds(selection)

[jacwright]

We could also add an example for a custom use-case.