Add ColorBrewer palettes

[jgabry]

Closes #177

This PR allows ColorBrewer palettes to be used with bayesplot::color_scheme_set().

Basically, if a user specifies color_scheme_set("brewer-X"), where X is the name of a ColorBrewer palette recognized by RColorBrewer, then bayesplot will use RColorBrewer::brewer.pal(n = 6, name="X") to get the colors.

I also cleaned up and rearranged some internals in the bayesplot-colors.R file and changed the roxygen to use markdown, so it may seem like more changed than actually did. The only change that changes any behavior is in these lines in the internal scheme_from_string() function:

bayesplot/R/bayesplot-colors.R

Lines 251 to 258 in ac6f8ae

	} else if (identical(substr(scheme, 1, 7), "brewer-")) {
	# user specified a ColorBrewer scheme (e.g., "brewer-Blues")
	if (!requireNamespace("RColorBrewer", quietly = TRUE)) {
	stop("Please install the 'RColorBrewer' package to use a ColorBrewer scheme.",
	call.=FALSE)
	}
	clrs <- RColorBrewer::brewer.pal(n = 6, name = gsub("brewer-", "", scheme))
	x <- setNames(as.list(clrs), scheme_level_names())

Example Usage

color_scheme_set("brewer-Spectral")
mcmc_trace(example_mcmc_draws(), pars = "sigma")

[codecov-io]

Codecov Report

Merging #190 into master will decrease coverage by 0.04%.
The diff coverage is 97.1%.

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   99.36%   99.31%   -0.05%     
==========================================
  Files          30       30              
  Lines        4083     4088       +5     
==========================================
+ Hits         4057     4060       +3     
- Misses         26       28       +2

Impacted Files	Coverage Δ
R/bayesplot-ggplot-themes.R	94.59% <ø> (ø)	⬆️
R/bayesplot-colors.R	98.51% <97.1%> (-1.49%)	⬇️

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[tjmahr]

ggplot2 will take nine character ones where the last two set the alpha level. Some palettes emit colors with these extra characters.

scales::viridis_pal()(4)
#> [1] "#440154FF" "#31688EFF" "#35B779FF" "#FDE725FF"

Not sure we need to accommodate this though.

[jgabry]

Oh, I didn't know that. I'm not sure we need to accommodate it either, although I'm not opposed. I suppose adding support for that can be a separate issue if we every want to do it or get a request.

[tjmahr]

Take a look at my changes and feel free to merge.

[jgabry]

Thanks @tjmahr. Merging this now. (Travis checks are good except for a note about the hexbin package but that’s taken care of by my other PR #187)