Simplify font fallbacks and use Maps for configuration

- Fixes #25 and #13
- Superseeds #16

The map seems like a better way to see what is loaded in one set:

```scss
$_o-fonts-families: (
	BentonSans: (
		font-family: 'BentonSans, sans-serif',
		variants: (
			(weight: lighter, style: normal),
			(weight: normal,  style: normal),
			(weight: bold,    style: normal)
		)
	),
	MillerDisplay: (
		font-family: 'MillerDisplay, Georgia, serif',
		variants: (
			(weight: normal, style: normal),
			(weight: bold,   style: normal)
		)
	),
	Clarion: (
		font-family: 'Clarion, Georgia, serif',
		variants: (
			(weight: normal, style: normal),
			(weight: bold,   style: normal),
			(weight: normal, style: italic)
		)
	),
	FinancierDisplay: (
		font-family: 'FinancierDisplay, serif',
		variants: (
			(weight: normal, style: normal),
			(weight: normal, style: italic)
		)
	),
	FinancierText: (
		font-family: 'FinancierText, serif',
		variants: (
			(weight: normal, style: normal),
			(weight: normal, style: italic)
		)
	)
);
```

Also helped discover [a bug in LibSass](https://github.com/sass/libsass/issues/741) when trying to make the module compatible with it.