Add some badges to README

They are always nice to have.

README.md

@@ -1,5 +1,11 @@
-Plorth/TypeScript
-=================
+# Plorth/TypeScript [![travis][travis-image]][travis-url] [![coveralls][coveralls-image]][coveralls-url] [![npm][npm-image]][npm-url]
+
+[travis-image]: https://img.shields.io/travis/RauliL/plorth-ts/master.svg
+[travis-url]: https://travis-ci.org/RauliL/plorth-ts
+[coveralls-image]: https://coveralls.io/repos/github/RauliL/plorth-ts/badge.svg?branch=master
+[coveralls-url]: https://coveralls.io/github/RauliL/plorth-ts
+[npm-image]: https://img.shields.io/npm/v/plorth-interpreter.svg
+[npm-url]: https://npmjs.org/package/plorth-interpreter
 
 Implementation of [Plorth](http://plorth.org) interpreter written in
 [TypeScript](https://www.typescriptlang.org), so that it can be run with