Adds modules and configuration to produce an ES module bundle.

[qubyte]

I'd like to integrate ibankit with a project using ES modules. This PR:

• Adds rollup config to take the TS source and spit out a single module file (with ES5 content).
• Updates the build script to run rollup after running tsc.

This PR places the ES build alongside the other build artefacts in the. ./lib folder.

This PR is intended to broach the subject. If this PR is welcome, then it may be better to place bundles in a dist folder. There may also be an argument for including a single UMD bundle too, so all possible module systems (or lack thereof) are addressed:

ibankit-js/
└── dist/
    ├── ibankit.umd.js
    └── ibankit.esm.js

That said, I don't know how to express the location of the UMD module in package.json when there's already a set of CommonJS. modules referenced by the main field (ESM is easy because module was unused).