Add ability to select multiple list items at a time for formatting or deletion

[benlammers]

Current behaviour: As of now you can only select one list item at a time, this can make formatting and deleting items tedious

Expected behaviour: Add ability to select multiple list items at a time and apply styles and formatting to all at once or delete all

[khanakia]

I need the same features. It's irritating at the moment

[jwnukoski]

Seems to relate to not having contenteditable="true" in top level ul or ol element. I'm not sure if this creates any regressions. It seems to work well within Directus CMS. I've created a PR here #77.