Implement text expansions

[bantic]

• Trigger is "space"
• "*" -> ul>li
• "1" and "1." -> ol>li
• "##" -> h2
• "###" -> h3

Adds editor#registerExpansion which takes an expansion.
An expansion has these keys:

• trigger the keycode of the triggering key event. All of the existing ones use 32 ("space")
• text the text that must match to the left of the cursor. Currently this only looks for text matching from the beginning of the section to the cursor. In the future it wouldn't be hard to modify it to handle in-context text, so you could bold text by adding "**" on either side of a word, perhaps
• run a method that will get passed the editor instance when this expansion is triggered

fixes #87

[bantic]

cc @ErisDS This was something you had asked about a while back. It's finally landed!