Fix #4 with babel, add npm script build

Also needed to modify index.ts with an import babel-polyfill.
`npm run build` now compiles from toolchain/, and then runs the babel
build process.

Author: ning-y

.babelrc

@@ -0,0 +1,3 @@
+{
+  "presets": ["es2015", "stage-2"]
+}

package-lock.json

@@ -7,8 +7,9 @@
     "@types/common-tags": "^1.4.0",
     "@types/estree": "0.0.37",
     "@types/invariant": "^2.2.29",
-    "astring": "^1.2.0",
     "acorn": "^5.1.2",
+    "astring": "^1.2.0",
+    "babel-cli": "^6.26.0",
     "commander": "^2.14.1",
     "common-tags": "^1.4.0",
     "invariant": "^2.2.2"
@@ -17,12 +18,21 @@
   "bin": {
     "slang": "./slang.js"
   },
+  "scripts": {
+    "build": "tsc; babel toolchain -d lib"
+  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/ningyuansg/slang.git"
   },
   "bugs": {
     "url": "https://github.com/ningyuansg/slang/issues"
   },
-  "homepage": "https://github.com/ningyuansg/slang#readme"
+  "homepage": "https://github.com/ningyuansg/slang#readme",
+  "devDependencies": {
+    "babel-core": "^6.26.0",
+    "babel-polyfill": "^6.26.0",
+    "babel-preset-es2015": "^6.24.1",
+    "babel-preset-stage-2": "^6.24.1"
+  }
 }

package.json

@@ -1,3 +1,4 @@
+import 'babel-polyfill'
 import { Context, Scheduler, SourceError, Result } from './types'
 import { evaluate } from './interpreter'
 import { InterruptedError } from './interpreter-errors'