Add option 'double down'

[swapagarwal]

After receiving an initial two cards, the player should also have an option of going double.
In double down, the player can double the initial bet in exchange for committing to stand after receiving exactly one more card.

[vishalkuo]

Will take a look and submit pull request when possible

[swapagarwal]

Hey @vishalkuo, @Randyfreak has already implemented this functionality. You can go for another one if you like! Thanks.

[UshalNaidoo]

Hey @swapagarwal, If this is complete can you mark it as done please. Thanks. I created a PR for it. Thanks. Sorry for being away, had a lot of home stuff going on. Am back now.