feat(docs-custom): add config argument

[jgroth]

I would like to have access to the logger when using the docs-custom output target, at the moment only the docs generated by Stencil is given to the generator function.

Sending only the logger to generator seems a bit weird and since the logger is available on config I added that as an argument instead. There might be other stuff on config that also could be of use to anyone using the docs-custom output target.

If there is another way to do this, please let med know! 🙂
I'm also not sure if there is anything else I need to update, all the tests pass at least! 🙂

[adamdbradley]

looks good, thanks!