Obtain license name from package.json instead of hardcoding (#129)

The `license` field in the `package.json` file should contain the license name. That should be used instead of hardcoding it to 'MIT'.

Author: dhruvkb

rollup.config.js

@@ -24,7 +24,7 @@ function getAuthors(pkg) {
 const banner = `/*!
   * ${pkg.name} v${pkg.version}
   * (c) ${new Date().getFullYear()} ${getAuthors(pkg)}
-  * @license MIT
+  * @license ${pkg.license}
   */`
 
 // ensure TS checks only once for each build