Provide index.d.ts

Hello @akiomik ,
Please, provide declarations file for the package. It will be handy to use your library in TypeScript.

Links:
- [Default chrome declarations](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/chrome/index.d.ts) - as an example;
- [Official Typescript Manual](https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html) - you'll probably will need to provide path to declarations like this:
```
{... ,
"types" : "index.d.ts",
... }
```

Regards.