chore: add clean and check before dist packaging

bin/check-if-clean

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+if [[ $(git diff --shortstat 2> /dev/null | tail -n1) != "" ]]; then
+  echo "Working directory is dirty!"
+  exit 1
+fi

package.json

@@ -111,9 +111,9 @@
     "add-licenses": "./bin/add-licenses",
     "build": "gulp build",
     "check-licenses": "./bin/check-licenses",
-    "clean": "rm -rf build && git clean -X -f",
+    "clean": "rm -rf build && rm -rf dist && git clean -X -f",
     "dev": "gulp build && cd build && npm link && cd .. && gulp watch",
-    "dist": "gulp dist",
+    "dist": "npm run clean && ./bin/check-if-clean && gulp dist",
     "fix-format": "node_modules/.bin/tslint -p . --fix && node_modules/.bin/tsfmt -r",
     "integ": "./test/integ/run",
     "lint": "gulp lint",