rename container's $padding argument to $padding-x

[marcelmoreau]

At first, this confused me as the _variables.scss file also has a $padding variable. IMO, $padding-x is clearer and could reduce others' potential confusion. I also modified the notes for this.

[mbfitz]

+1 i like the distinction here, much clearer where the value is coming from!

[khamer]

Can we use the existing convention of $h- or $v-, i.e. $h-padding instead? We're not using anything like $padding-x right now, and $h-padding or $horizontal-padding feel more consistent and clear to me.

[mbfitz]

@khamer @marcelmoreau I'm ok if it changes to $h-padding rather than $padding-x for consistency.

[marcelmoreau]

renamed it