Semver-based migrations

- Put migrations into a separate file: migrations.js
- Add semver, for more specific version-based migrations.
- Change `state.saved.version` to semver app version.

package.json

@@ -32,6 +32,7 @@
     "network-address": "^1.1.0",
     "prettier-bytes": "^1.0.1",
     "run-parallel": "^1.1.6",
+    "semver": "^5.1.0",
     "simple-concat": "^1.0.0",
     "simple-get": "^2.0.0",
     "srt-to-vtt": "^1.1.1",