fix(github): allow multiple accounts

[shreyas1599]

Fixes #168

• The authorization url has a login prop which I used. This will allow logging in to multiple accounts.
• Regarding preventing creation of duplicate accounts, I added a method but if the user enters an email instead of the username, it won't work. (Not sure if this is the best way to do it. @pd4d10 please suggest if there's a better way)