Expand options for visibility classes

[mattada]

Currently, we have classes for md-hide-down and lg-only, which are pretty much the same thing.

Suggestion: add classes for lg-hide' and md-show-downThelg-hidewould be in the family of thesm-andmd-hide classes. Themd-show-downwould be the mirror ofmd-hide-down`.

Thanks for considering my suggestion for the july 2017 release of Gingergrid. If selected, I will wear this issue as a badge of honor, as represented in this interpretive dance.