v3.0.0

• docs: indicate postcss installation 84cf545
• refactor: tweak tests 79d1fa3
• refactor: migrate postcss to v8 6da0926
• Bump ava from 3.12.1 to 3.13.0 981f74b
• Bump postcss from 7.0.32 to 7.0.34 d2d2f4c
• Bump xo from 0.33.0 to 0.33.1 903416a
• Bump ava from 3.11.1 to 3.12.1 1230e67

v2.0.0...v3.0.0