Enable loading control addition after map initialization

In some cases, adding a control via options passed to map create is not an option. Hence, the ability to add the loading control via map.addControl should be available. This achieves that.

Currently, this includes code that doesn't make sense without the changes made as part of [Improved Loading State Tracking](https://github.com/ebrelsford/Leaflet.loading/pull/1). However, it would not be difficult to change that if ex post facto control addition is desired without the loading state hash.