Allow easier access to header / body font families for overrides

[jaredly]

Looks like the current best way to get the headerFontFamily into something that's not an hX or similar is this, taken from gatsby:

            css={{

              fontFamily: typography.options.headerFontFamily.join(`,`),
            }}

it would be great to provide a helper method that would make it feel first-class

[KyleAMathews]

good idea! Now that Gatsby is at a 1.0, planning on circling back to Typography.js sooner than later to get it over the hump to a 1.0.