Allow linear model formula to extract variables from calling scope.

[adamboche]

Resolving #3381, by adding an eval_env argument to from_formula, based on https://patsy.readthedocs.io/en/latest/API-reference.html#patsy.dmatrix.

[lucianopaz]

First of all, thanks so much for the contribution! I think it's great! I have some comments though.

This change should be noted in the RELEASE-NOTES.md features section.

[lucianopaz]

@adamboche, thanks so much!