added webmail-linker.js

#### About

This is a JavaScript implementation of [Webmail Linker](https://github.com/thomasbachem/webmail-linker). It can be used as a node.js module or in a browser.

##### What's Webmail Linker?

 > Many websites require an email double opt-in during user registration. This project wants to fix the resulting break in user flow when users have to manually switch to their email providers' website to receive the confirmation email.

 > With Webmail Linker, you can offer a direct link to the user's webmail interface. In addition to the webmail URL, you'll get the name of the email provider and a favicon URL, so you can offer a button like "Check your email on [name and icon of the provider]".