Introduction of collectors i.e. custom stream processors

This release adds a new Collector type.
It allows the creation of custom stream reductions.
See the README.md file for further details.

I'm also removing the alpha tag from the release as it seems the code and design has reached a degree of maturity now. I will still be making changes and additions but as a continuation of where the code is now (rather than a discontinuation as with in some cases in past releases).