Support extensible loaderContext

[gusvargas]

There have been some issues related to the lack of loaderContext provided:

#10 (loadModule)
#25 (target, minimize, and resourceQuery)

We have a suite of custom webpack plugins and loaders which expose and consume custom APIs. Rather than forking the thread-loader to support our use cases, what do you think about making the IPC bridge extensible so that new capabilities can be added without bloating the "core" context?