Limit font-size effects on heavily nested elements

Heavily nested lists and block quotes could result in *really* tiny font
sizes. We now use `font-size: inherit` on certain nesting combinations,
and there's an included test case for it.
Increased `line-height` (thanks Thomas) (v0.1.2)

If the `line-height` is too small inline `<code>` blocks would run into
each other. As Thomas suggested, a `line-height` of `1.3em` fixes this,
and improves document flow too.

Additional minor spacing tweaks were made to match the slightly taller
`line-height` (such as increasing inter-paragraph spacing).

Tests for `<code>` tags and for long document flow are now included.