Loader Expansions #206

See this PR comment.

The loader introduced by #190 is built directly into the init process, but could be expanded to allow the implementer finer-grained control of when to display the loader. Specifically, if the div that will eventually house ULabel exists, they should be able to call a function that immediately attaches the loader, allowing the display to span the time of their implementer's application loading logic, as well as the loading logic internal to ULabel itself.

If properly generalized, the loader could be re-applied to the ULabel div when doing "hot reloads", or computationally expensive annotation transforms.