Add display: inline-flex; to flexbox rules

[CygnusRoboticus]

Closes #221

[viktorbijlenga]

I think .flex-inline would make more sense as the selector name. Just my feeling that it would be easier to add -inline to change between the different displays states.

[CygnusRoboticus]

@viktorbijlenga I based .inline-flex off of the existing .inline-block. I don't know if there was reasoning behind inline-block vs block-inline, but I'd prefer consistency among display: -type selectors.

[viktorbijlenga]

Sounds good