Make !sell and ! buy command

[Billason]

like tf2automatic we need !sell or !buy command

[xmwx38]

@Billason Check out this comment - #50 (comment)

Also, be sure to check out this chapter from a forked repo of node-steam-guide - https://github.com/Steam-Bot-Basics/node-steam-guide/tree/master/Chapter%203%20-%20User%20Interaction/Chapter%203.2%20-%20Chatting%20With%20Friends