Request: allow set of font before page start

[johnnyshields]

Currently calling pdf.font('font_name') before creating first page throws Prawn::Errors::NotOnPage

However, pdf.fallback_fonts works fine in the same place. Would be nice to be able to call pdf.font here.

[practicingruby]

We need to more comprehensively think about what to allow / not allow before a page is created. But I agree that we should at least be consistent, and for things that aren't specifically "page related" it may make sense to allow setting them in advance.