Adds a sapper extract CLI command, which scrapes the server to run as a static website starting at the site's root.

TESTED=Basic unit test ensuring relevant routes are added.

package.json

@@ -19,11 +19,15 @@
   "dependencies": {
     "chalk": "^2.3.0",
     "chokidar": "^1.7.0",
+    "compression": "^1.7.1",
     "escape-html": "^1.0.3",
+    "fs-extra": "^5.0.0",
     "mkdirp": "^0.5.1",
+    "node-spider": "^1.4.1",
     "relative": "^3.0.2",
     "require-relative": "^0.8.7",
     "rimraf": "^2.6.2",
+    "walk-sync": "^0.3.2",
     "webpack": "^3.10.0",
     "webpack-hot-middleware": "^2.21.0"
   },