Add tests using context in Plugin Context Processors

For issue #1199
plugin context processors now take a context argument

Amongst other things this will give them access to the request object.

Fixes issue #1199
Add docs for Plugin Context Processor change

(The change being the new argument passed to them so they can use the context).
Merge pull request #1833 from foobacca/context_for_plugin_context_proc

Context for plugin context proc