output formatting

[manncz]

From @adamSales https://github.com/adamSales/rebarLoop/issues/3#issue-1584262683:

Nicer output--right now, loop() just returns two unlabeled numbers with no formatting. We should write a print.loop function that gives output in the style of, say, estimatr, or even lm, with labeled estimate, standard error, hypothesis tests, confidence intervals

Oh yeah, I also think there should be optional output like, say, R^2 from the random forests or whatever. we could go to town

[manncz]

d830dc7 creating a print and confidence interval function