Modernize boxplot and add a number of enhancements

[mwaskom]

This PR continues the work in #2429, addressing boxplot. As in the related PRs, boxplot now runs through the common variable assignment and axes setup and has gained the new native_scale and formatter parameters. I won't repeat all the details here.

The PR also adds a number of new features:

Unfilled boxes

With fill=False, no box patch will be drawn and colors will be mapped to the line elements:

ax = sns.boxplot(tips, x="day", hue="day", y="total_bill", fill=False)

Gapped dodging

With gap > 0, boxes will be shrunk along the orient axes after dodging, leaving a gap between them:

sns.boxplot(tips, x="day", y="total_bill", hue="sex", gap=.1)

Closes #1455

Single-argument linecolor parameterization

The new linecolor parameter allows to color of all line elements in the boxplot to be changed in one step

sns.boxplot(tips, x="size", y="total_bill", color="w", linecolor="r")

Artist containerization

The artists that comprise each boxplot are now packaged in a BoxPlotContainer for easier post-plotting access

ax = sns.boxplot(flights_wide, fill=False, showcaps=False)
bxp = ax.containers[0]
bxp[5].box.set_color("g")
plt.setp(bxp[6], color=".2")
plt.setp(bxp[7].whiskers, color="r")

Increased customizability with matplotlib prop dicts

Any customizations passed through matplotlib keyword arguments are now applied on top of the changes that seaborn makes to apply its boxplot style.

Default flier marker

The default flier marker now follows the matplotlib rcparams so that it can be globally customized.

Other issues

The improvements to variable assignment fix #2756.

[codecov]

Codecov Report

Merging #3294 (e10392d) into master (e1e36cb) will decrease coverage by 0.04%.
The diff coverage is 97.13%.

❗ Current head e10392d differs from pull request most recent head 3786b8f. Consider uploading reports for the commit 3786b8f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3294      +/-   ##
==========================================
- Coverage   98.38%   98.35%   -0.04%     
==========================================
  Files          77       77              
  Lines       24712    24942     +230     
==========================================
+ Hits        24313    24531     +218     
- Misses        399      411      +12     

Impacted Files	Coverage Δ
seaborn/categorical.py	97.51% <95.54%> (-0.33%)	⬇️
tests/test_categorical.py	99.55% <98.09%> (-0.19%)	⬇️