Chore: Add eslint peer-dependency

This was removed in a prior version because other linting tools ended up
depending on helper functions within eslint-plugin-prettier and did not
want to inadvertently install eslint as a transitive dependency. Now that
those helpers have been extracted into a separate package those other
linting tools should depend upon that instead of eslint-plugin-prettier.

package.json

@@ -31,6 +31,7 @@
     "prettier-linter-helpers": "^1.0.0"
   },
   "peerDependencies": {
+    "eslint": ">= 5.0.0",
     "prettier": ">= 1.13.0"
   },
   "devDependencies": {