Snippets UI

Building on top of @dougdroper's mastalk Scribe plugin:

* Adds Snippets dropdown menu
* Adds enabled/disabled state for buttons
* Adds closeOnClick functionality to Popover component (tests are coming)
* Adds all snippets
* Restricts inserting snippets within another tag (can only be inserted on a new line)
* Other miscellaneous improvements/fixes
But what about rendering the extra `caption` and `|===`?