docs: add onTextInput to README, add example to docs

[bantic]

As mentioned in Gitter by @martinklepsch, the readme should mention onTextInput here: https://github.com/bustlelabs/mobiledoc-kit#editor-api
And inline documentation should be added to the method definition in editor.js

[mixonic]

We still need some inline docs.

Aside: Is there a benefit to having the matcher pass as a property on options? Perhaps the key pressed should simply be passed as an argument to the hook, unifying it with other hook argument signatures.