Add bootstrapped fits for atmospy.regplot

[dmcc]

Is your feature request related to a problem? Please describe.

On some datasets, a couple outliers can wreck the best fit line in atmospy.regplot.

Describe the solution you'd like

We'd like the option to show a bootstrapped fit with confidence bars rather than estimating the fit on the whole dataset.

Describe alternatives you've considered

This might(?) be doable today by passing the right keyword arguments to regplot but it's a common enough thing that we should at minimum document how to do this.