Adjusted typography system

After using in practice, there were some problems with the type scale
on a content based site, so I’ve adjusted it. The base type is 18px,
which sounds big, but really it isn’t for where our design work is
going. If you need to change your child scheme type scale size, you
just need to declare the html font size which will adjust the base, but
the pixel fall back will not be adjusted as this is a sass calculation
based on a variable in _typography.scss