Add is-relative helper

[lucperkins]

I've gotten bitten on a few projects where I expected sections to act more like containers, i.e. to set horizontal limits to what's inside of them. This change makes sections have what I take to be more predictable behavior.

[jgthms]

I understand the idea behind this change, but it might break several setups, if users rely on the fact that sections are not positioned, and that they position elements in reference to the body, not the section.

[lucperkins]

Ah, okay, that makes sense. I wasn’t fully aware of the range of uses. I’ll modify the PR to include some documentation changes instead.

[micovi]

How about adding .is-relative helper class that we can use on elements where we need this behavior?

[lucperkins]

Okay, I've updated this PR to supply an is-relative helper class plus docs. I totally understand if you feel like this either isn't useful or should be part of a larger feature set. If the latter is the case, let me know and I'd be glad to help out.