Better highlight and document CSS-free customizations

[gadenbuie]

From recent Twitter discussions, a clear strength of this package is the ability to adjust slide styles without using CSS. Secondarily, discoverability is also important -- knowing where to look to find out what you can change is important.

Current documentation focuses on discoverability and assumes some familiarity with CSS. Some improvements:

• Better highlight that pkg accommodates easy CSS-free changes
• Add section or table of parameters (or table in docs, vignettes) with parameter and CSS-level (no CSS required, low CSS, intermediate, high, etc.)
• Make sure documentation gives an idea about what kinds of CSS params are expected in each function args, bonus points for examples

[gadenbuie]

Hoping v0.1.1 mostly resolves this...