Fixes #53

Fixes #50

Fixes #41

Fixes #44

Avoids note for long argument length and allows for more consistent checking if supplied arguments are default values

Closes #33

E.g. primary_color and secondary_color in style_duo() become --primary and --secondary

Closes #32

And add tests

Closes #28

header_background_padding is now learned from the slide padding so that left and right margins match

Now, font size is inherited from `base_font_size` which should be in absolute units.

Also allow NULL or NA for template arguments but change default to NULL

- Also reorder code params

Add "Font" to documentation: "Text Font Size"

Header fonts are generally not the same font as text font, so linking the two requires tweaking other properties (padding, etc.) of the header.