Fixes #442 - space before adjacent `nav`.

Fixes #442 - space before adjacent `nav`.

As it turns out Android stock browser <= 4.3 needs a space
between `+` and `nav`, e.g. `div+ nav{}`.

It's off by default and can be turned on with:
* `--compatibility +selectors.adjacentSpace` or
* `compatibility: { selectors: { adjacentSpace: true } }`

See http://codepen.io/anon/pen/QwgLVv