add package.json repository, homepage

[makepanic]

This makes it easier to find the source via npm repositories.

Problem

When viewing twemoji-parser via npm or yarn, it's hard to find the package source.

Solution

I've added package.json metadata to point to this git repository.

Result

Npm and yarn will point to this git repository.

[CLAassistant]

All committers have signed the CLA.