Support Connection String URI Format for mongo shell

Hi to all!)

Usage:
mongo mongodb://username:password@host:port/database
mongo mongodb://username@host:port/database

If you use env MONGO_URL=mongodb://username:password@host:port/database var for you project u can to use:

mongo $MONGO_URL

Pull request for native support:
https://github.com/mongodb/mongo/pull/995

Bash helper:
https://github.com/aggregable/mongourl