Issue 170 custom font wording

This PR attempts to resolve #170.

There's one side change here to help prevent link rot and I expect pure has their own view on this, so I'm happy to redo the request if required.

sectionHeading generates the ID based on the title text, this PR chooses to change that title text from `Using Grids with Your Custom Fonts` to `Using Grids with Your Font-Family`. As a result, links to the original heading will break.

To resolve this, commit a7138cdff3d6c759fcce00c32fcfb88ae5964948 adjusts sectionHeading to take the id as an optional argument. Since sectionHeading already had ordered arguments (not accepting arguments as an optional object) I thought the best method was to refactor sectionHeading take all the optional arguments as an object.

No other calls to sectionHeading were using the existing optional arguments, and other similar functions already accepted optional arguments as an object.

Finally, commit 256870151bc351c6b2280cbe33c7ee96415f8caf changes references to `font-family` instead of `custom fonts` and changes the section heading without changing the existing link. With one further grammatical fix in ff1f0b094e5398cfd2031e2db315dba0404623b7.