Add mailcheck.js

Added [mailcheck.js](https://github.com/Kicksend/mailcheck#mailcheckjs)

Files taken directly from upstream:
https://github.com/Kicksend/mailcheck/blob/master/src/mailcheck.js
https://github.com/Kicksend/mailcheck/blob/master/src/mailcheck.min.js

I think it passes the popularity threshold easily: as of right now, 160 watchers, 3760 stars and 240 forks.

[Hacker News thread](https://news.ycombinator.com/item?id=3731033)