Flexbox helpers

[mrm]

This is a new feature.

When using Bulma I often use is-flex but the modifiers you'll want to use are missing such as justify-content: space-between. I found a couple of issues/PRs such as the most recent #3008 which suggest adding flexbox helpers.

Proposed solution

I've added most flexbox options such as is-justify-content-space-between. Perhaps naming can be improved as some are quite long (the suggested is-justified-space-between or {has-,is-}justify-between are probably be better), and perhaps some hardly used values can be removed.

So it's not done yet, but wanted to get some feedback early on.

Tradeoffs

Testing Done

None.

Changelog updated?

No.

[jgthms]

I think sticking to the exact property name is better in this case, to avoid ambiguity.
Also, can you rename the file to flexbox.sass?
Thanks.

[Rome00]

flex helpers need responsive options too like -mobile, -tablet, -desktop ..... so naming would be quite long .is-justified-space-between-tablet .is-justify-between-tablet will be better or
something like .is-flex-between for justify-content and .is-vflex-between for align-items.

[service-paradis]

This PR would fix #3008, #3078 and #3071