Add apply and pipe methods to Grid objects for fluent customization

[mwaskom]

These two methods accept an function that operates on a Grid object to do additional customization of the plot. The methods are very similar, but apply returns the Grid it is attached to while pipe returns the value from the passed function. The objective is to make it easier to do arbitrary customization without catching the return value and switching to an imperative programming style.

Example:

def slopeline(grid, slope, **kwargs):
    grid.ax.axline((0, 0), slope=slope, **kwargs)

(
    sns.relplot(tips, x="total_bill", y="tip")
    .apply(slopeline, slope=.2, color=".6")
    .set_axis_labels("Total bill", "Tip")
)

Additionally, fixed two methods (tight_layout and refline) that were not returning self.

[codecov]

Codecov Report

Merging #2928 (8b78045) into master (f581300) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2928   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files          69       69           
  Lines       22866    22890   +24     
=======================================
+ Hits        22464    22488   +24     
  Misses        402      402           

Impacted Files	Coverage Δ
seaborn/axisgrid.py	97.18% <100.00%> (+0.01%)	⬆️
tests/test_axisgrid.py	99.37% <100.00%> (+<0.01%)	⬆️