Include type declarations in the npm packge

[lantw44]

Since this package is written in TypeScript, it will be nice if you can provide .d.ts files in the published npm package to allow other projects to reuse your type declarations. It should be as simple as adding a single line to tsconfig.json:

diff a/tsconfig.json b/tsconfig.json
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -7,6 +7,7 @@
         "target": "es6",
         "module": "commonjs",
         "noImplicitAny": true,
+        "declaration": true,
         "moduleResolution": "node",
         "baseUrl": "./",
         "paths": {

This package does not build on my machine, and I cannot test if the patch works for you.