build(deps-dev): bump posthtml from 0.15.2 to 0.16.4

Bumps [posthtml](https://github.com/posthtml/posthtml) from 0.15.2 to 0.16.4.
- [Release notes](https://github.com/posthtml/posthtml/releases)
- [Changelog](https://github.com/posthtml/posthtml/blob/master/changelog.md)
- [Commits](posthtml/posthtml@v0.15.2...v0.16.4)

---
updated-dependencies:
- dependency-name: posthtml
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -33,7 +33,7 @@
     "ava": "^3.5.0",
     "np": "^7.4.0",
     "nyc": "^15.0.0",
-    "posthtml": "^0.15.1",
+    "posthtml": "^0.16.4",
     "posthtml-render": "^1.2.0",
     "xo": "^0.39.1"
   }