Add on_demand engine configuration #42
Description
Some engines might be used only rarely in which case it makes sense to make them run-time optional that is initiated only on demand. on_demand flag would signal this and when such a flag is found lazy wouldn't not instantiate the engine but wait for a signal to do so. Signals would be 1) requests to engine's internal URL 2) requests to analyze one of the languages which the engine declares to process (this obviously means that all-language engines could not be on demand)