Add Tailwind 2’s peer dependencies to preset

[knowler]

Follow up to #40

Tailwind 2 has two peer deps:

1. PostCSS 8
2. Autoprefixer 10

In reality, only the missing PostCSS dep will cause building to fail, but I thought I should just add both to be safe.

[knowler]

I think that maybe the proper solution to the increase in Node requirement is to also modify the engines property in package.json. This would require a new method on the Preset class.