Provide commonjs and esm bundles as entry points

In this diff I replaced lib entry point with two bundles

main: ./dist/remarkable.cjs.js
module: ./dist/remarkable.esm.js

lib will be rewritten with esm.

Bundles has a few benefits
- faster startup time in node
- faster bundling
- internals are hidden and changing them will not affect users

package.json

@@ -50,13 +50,15 @@
     "index.js",
     "lib"
   ],
-  "main": "index.js",
   "bin": "./bin/remarkable.js",
+  "main": "./dist/remarkable.cjs.js",
+  "module": "./dist/remarkable.esm.js",
   "engines": {
     "node": ">= 0.10.0"
   },
   "scripts": {
-    "test": "make test",
+    "build": "make rollup",
+    "test": "make rollup && make test",
     "test-ci": "nyc mocha -r esm -R spec --bail",
     "coverage": "yarn add coveralls@2 && nyc report --reporter=text-lcov | coveralls"
   },

rollup.config.js

@@ -1,10 +1,31 @@
+import path from 'path';
 import nodeResolve from 'rollup-plugin-node-resolve';
 import commonjs from 'rollup-plugin-commonjs';
 import { terser } from 'rollup-plugin-terser';
+import pkg from './package.json';
 
 const name = 'Remarkable';
+const external = id => !id.startsWith('.') && !path.isAbsolute(id);
 
 export default [
+  {
+    input: ['./lib/index.js', './lib/linkify.js'],
+    output: { file: 'dist/cjs', format: 'cjs' },
+    external,
+    plugins: [
+      commonjs(),
+    ]
+  },
+
+  {
+    input: ['./lib/index.js', './lib/linkify.js'],
+    output: { dir: 'dist/esm', format: 'esm' },
+    external,
+    plugins: [
+      commonjs(),
+    ]
+  },
+
   {
     input: './lib/umd.js',
     output: { file: 'dist/remarkable.js', format: 'umd', name },
@@ -13,6 +34,7 @@ export default [
       commonjs(),
     ]
   },
+
   {
     input: './lib/umd.js',
     output: { file: 'dist/remarkable.min.js', format: 'umd', name },