Support existing tasks

Currently any existing task in the main gulpfile.js (containing the call to gulp-hub) would be overwritten by subtasks with the same name. This will preserve the existing task and execute the subtasks as dependencies. 

Needed that when building all subprojects, the main gulpfile then manages the resulting artefacts of those subprojects, e.g. combine them to a single project. Maybe useful for others too...

Added that use case to the examples.