Add scatterplot function

[mwaskom]

This PR adds a scatterplot function that can set the color, size, and marker of the points based on semantic variables. Simple obvious example:

sns.scatterplot(
    x="total_bill", y="tip",
    hue="sex", style="smoker", size="size",
    data=tips,
)

More examples to come.

Possible functionality to be added in future development:

• Jittering
• Automatic point opacity for large datasets
• Aggregation and estimation similar to lineplot

Ultimately, this should also replace the underlying scatter plotter in regplot (and therefore lmplot), and perhaps more pressingly, should be used by default in pairplot and jointplot.

In principle the API here will work fine in FacetGrid but, similar to the case with the categorical plots, the ordering and limits for the various semantic variables will need to be explicitly defined so it can be shared across the facets. There should be a higher-level function similar to factorplot to draw these plots onto a FacetGrid. It will need a name. One would be facetplot but that is very easily confused with factorplot.

The presence of this function will make it awkward that size/style semantics cannot be used in the categorical scatterplot functions (stripplot/swarmplot), but adding them would probably be too disruptive.

Closes #315

To Do:

• Handle to-be-implemented functionality
• Tests
• API documentation
• Gallery examples

[codecov]

Codecov Report

Merging #1436 into master will increase coverage by 0.15%.
The diff coverage is 98.7%.

@@            Coverage Diff             @@
##           master    #1436      +/-   ##
==========================================
+ Coverage   94.17%   94.32%   +0.15%     
==========================================
  Files          15       15              
  Lines        4240     4300      +60     
==========================================
+ Hits         3993     4056      +63     
+ Misses        247      244       -3

Impacted Files	Coverage Δ
seaborn/basic.py	98.71% <98.7%> (+0.92%)	⬆️

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