Optional Parameters as Maps

[ajmueller]

This changes our functions and mixins to allow for optional parameters to be passed into the function or mixin as part of an optional map. Doing so will provide a couple advantages:

• Fewer parameters in calls when you want to change one optional parameter (see a column mixin example)
• Clearer syntax to know what optional parameters are being passed

That said, sometimes the syntax is more verbose than before in a way that may not be 100% desirable, such as the syntax for em functions, gradients, or aspect ratio. Some additional ideas to consider/discuss:

• Should we make the parameters for mixins such as gradient and aspect-ratio have only required parameters? I did this with the font-size mixin for simplicity and I think it might work well with others such as these two. NOTE: Since we have maps support now, I've opened #107 for consideration of changing how our gradient mixin works anyway.
• Documentation is sub-optimal due to limitations of sassdoc. Short-term: creative alternatives to explain the methodology behind optional parameters would be most welcome. Long-term: alternatives to sassdoc that give us more granular control over documentation would be great.

[ajmueller]

After some discussion with @elseloop, I changed the aspect-ratio and gradient mixins to have only required parameters. That leaves the em function. I think I currently would prefer to err on the side of leaving context as optional and use a map for options, even though context is the only option. Thoughts?

NOTE: As noted above, #107 has been opened to research additional possibilities with maps and the gradient mixin.

[elseloop]

@ajmueller, this all looks great and I'm not seeing any holes in testing. :+1:

In answer to your question about documentation, I've opened #109 to give us a place to explore that more fully.

[ajmueller]

Thanks @elseloop! I'll take a look at #109.

If nobody has any objections, I'll merge this PR.