Add h- and v- margin/padding Shortcuts

[matthewwithanm]

This adds four new mixins (h-margin, v-margin, h-padding, and v-padding) for setting only the values you want without duplicating them. This is preferable to using the CSS shorthand (which requires you to enter both the vertical and horizontal padding), and to separate -left and -right (or -top and -bottom) values, which requires you to duplicate the value.

[aebsr]

What do you think about offering an option that allows two values as well? so you could pass separate values for top vs the bottom. Similar to the .size() mixin seen here https://github.com/aebsr/bedrock/blob/master/mixins/declarations.less#L57

[matthewwithanm]

I think that I'd prefer to just use the -left and -right properties for that. I don't really mind the duplication of the properties—just the values—and using the regular css properties is more clear than a mixin which the reader might have to look up. Your call, though.

[aebsr]

If I'm extended a margin: 0, and want to later set it to margin: 1em 0 2em I'd have to still -top and -bottom. The purpose of your mixin is to remove the need to write 2 separate rules, so just allowing the mixin to optionally accept 2 vars only enhances that ability.

perhaps an edge case, but I've had to deal with situations like that kind of often. specially on nav and sidebar modules.

[matthewwithanm]

Yeah, I understand. I was just saying that I don't mind using -top and -bottom—as long as the values are different. IMO the only time writing both properties is a problem is when you have to duplicate a value.