Major updates to png outputs

• Depreciated build_thumbnail() and added build_png() as an improved replacement.
• build_png() fixes part of issue #15 so that the png aspect ratio matches that of the xaringan slides.
• build_png() also has options for changing the png density and building more than just the title slide (can build pngs of some or all slides using the slides argument).
• All documentation and examples updated to match new features.
• New example slides made to demonstrate new features.