Add a new modifier to Container

[chris-pearce]

Sometimes the Container layout module is part of a larger layout where that layout uses FlexBox, when this occurs there should be a way to change the Container to be display: flex because only a direct child of an element with display: flex can be a flex item.

We can achieve this via a modifier, for example:

.l-container--flexbox

[chris-pearce]

Fixed in #104.