Fixed: addTag no longer requires `insertContent` to be chainable
Created multi array controller and source list view
Added documentation (comments) and minor refactoring
Fixed a bug that would cause extra content to appear in multi arrays
Refactored source list view to render heading independently of listing

Added a bit of binding complexity… but...

Duplicate headings are much more unlikely (if not impossible)

Headings don't flicker when items are added to or removed from the list

Added a computed property for finding the root source listing
Added emberella-source-collection class name
Added class to base source list; prevented element ID from being copied to child lists
Moved source list item template definition to item class

Item view templates are no longer inherited from the containing source list view.

This maintains consistency with Ember's built in collection views.
Added `getFlattenedContent` method to multi array controller

Method flattens content across arrays/sub-arrays recursively to return a single array of items.
Improved computation of "active" selection

The "active" selection is typically the intersection of the current selection set and the current content array. That is, an item only appears actively selected when it is both in the selection set and the content.

A new `getActiveContent` method for determining the content array from which active selections must be a member of was added to allow multi array controllers and the selectable mixin to work together.
Reduced selection change event triggering
Made `arrangedContent` computation function length 2
Merge branch 'source-list' of github.com:realityendshere/emberella into source-list