Add initialization hook (`init`)
Merge branch 'hook-init' into develop
Register default content handlers before external content handlers
Content_Handler.render() passes deferred object to Custom handler

Resolution of custom handler passed transparently through to parent caller
CSS: Enable object visibility in viewer

* Objects, iFrames, etc.
Initial support for preloading item data
Wait until item data loaded before rendering template tags

* Add: `Content_Handler.load()`
Merge branch 'item-load-data' into opt-content-handlers
Unload item data in viewer when item changed
Merge branch 'viewer-unload' into opt-content-handlers
Return data package when matching content handler to item

* Optimize: Allow matching handler to send additional properties for item (formatted URI, etc.)
* Optimize: Standardized data type/structure returned by `Content_Handlers::match()`
Optimize: URI handling

* Cache items based on source URI
* Expand URI variants
Merge branch 'uri-handling' into opt-content-handlers
Cache content handler-specific properties for items
Merge branch 'opt-content-handlers' into develop
Optimize: Safer accessing of file contents
Merge branch 'opt-file-access' into develop
Remove unnecessary HTML comments in client output
Merge branch 'opt-client-comments' into develop
Fix: Load localized text into correct domain