Feature Request: guide_custom()

[teunbrand]

The idea here is analogous to annotation_custom() in that one might just provide a grob directly that will be treated as a (non-axis) guide for placement purposes. Internally, I imagine this just has guide_none()'s methods (i.e., no training, no transformation, no merging, no key glyph extraction etc.) for everything except the drawing part.
It might have a little but of functionality for displaying a title and has the same margin rules as other legend-like guides.