Corrects method for unsetting an elements ID.

While `el.id = null` works fine in PhantomJS it does not yield the expected results when test run in Chrome. If you set do `el.id = null` the `id` of the element will be the string "null".
Adds very basic extension support for Medium Editor.

Allows the user to register extensions that add a button to the editor and can implement various functions. Also includes an example in the README on how to use this to create a simple text highlighter. Also adds a fix so the Editor fails gracefully when the user defines the "buttons" option as empty array.
Merge pull request #196 from martinth/extension-api

Add basic extension API to Medium Editor