FEATURE REQUEST: add visibility classes

[cyberhck]

flexboxgrid doesn't have visibility classes

Since flexboxgrid is used heavily used for responsiveness, I believe visibility classes should be a part of core library.

There's a PR at kristoferjoseph#211 maybe can merge on this repo?

[imevro]

I agreed, will add in next beta

[SuEric]

I used https://github.com/vascocsilva/flexbox-grid-helper

However this feature shouldn't be difficult to implement. You should add it to this repo.

👍

[claviska]

Hoping to solve this with #4. It uses a combination of BS4-alpha's naming convention and BS4-beta's display utilities. Should cover most use cases.