add npm postinstall command to install archivebox via pip

package.json

@@ -6,10 +6,12 @@
   "repository": "github:ArchiveBox/ArchiveBox",
   "license": "MIT",
   "scripts": {
-    "archivebox": "./bin/archive"
+    "archivebox": "python3 -m archivebox",
+    "selfupdate": "python3 -m pip install --upgrade archivebox && npm install --no-audit --no-fund --loglevel=error",
+    "postinstall": "archivebox version --quiet || (python3 -m pip install archivebox && archivebox --version)"
   },
   "bin": {
-    "archivebox-node": "./bin/archive",
+    "archivebox": "python3 -m archivebox",
     "single-file": "./node_modules/.bin/single-file",
     "readability-extractor": "./node_modules/.bin/readability-extractor",
     "mercury-parser": "./node_modules/.bin/mercury-parser"