Add a concept of ignoring projects via extra hooks

Thank you for your work on a really helpful module!

I have a number of sites being monitored where I've either had to fork a contrib module, or continue using a discontinued module. In either case I really wanted to have the module update warnings suppressed when they were being generated due to these cases. To do this I introduced a concept of an ignore array and coded up the bare minimum that would be needed to achieve my goal. I introduced two hooks into your module.

In a moment, I'll attach the patch file, and would ask that you consider reviewing and merging into the main module if you agree with my approach.