Add exports to package.json to satisfy nodejs --experimental-module users

Fixes #206
This ensures that consumers using --experimental-modules with node 12+
will import the esm version of this package.

This package's export MUST have .mjs file extension... (because
otherwise the entire package must be specified as type: module)
This package's export MUST use exports field in the package.json
	see https://nodejs.org/api/packages.html#conditional-exports

Not sure how to test this with a bunch of different project styles..
webpack and other loaders _should_ be fine with importing from the
`module` field with a `.mjs` file extension?

Author: jamie-pate

package.json

@@ -20,6 +20,10 @@
   ],
   "main": "dist.es5/index.js",
   "module": "dist.es5m/index.js",
+  "exports": {
+    "import": "./dist.es5m/index.mjs",
+    "require": "./dist.es5/index.js"
+  },
   "es2015": "dist.es2015/index.js",
   "typings": "dist.es2015/index",
   "sideEffects": false,
@@ -28,7 +32,7 @@
     "build": "yarn build-modern && yarn build-es5 && yarn build-es5m",
     "build-modern": "tsc -p src",
     "build-es5": "tsc -p src --target es5 --outDir dist.es5",
-    "build-es5m": "yarn build-es5 -m es2015 --outDir dist.es5m",
+    "build-es5m": "yarn build-es5 -m es2015 --outDir dist.es5m; mv index.js index.mjs; mv index.spec.js index.spec.mjs",
     "build-watch": "yarn build-es5 -w",
     "clean": "rimraf 'dist*'",
     "lint": "eslint 'src/*.ts'",