New color manipulation functions

Need to add examples
Add color examples
Tweak examples, docs
More docs/examples
Incorporate code review feedback from @hadley
Proper NA handling; fix R CMD check
use with() instead of htmltools::withTags() to avoid R CMD check (false positive) notes
Return a color for NA; warn if color scaling causes NA
Fix a couple cases, add tests for NA warnings
Treat 'lon' as longitude column
Fix broken popups
Merge pull request #15 from rstudio/feature/color-funcs

Functions for easy coloring