Add expiration feature

[RickCogley]

Slack's users.profile.set API allows a profile parameter status_expiration which takes an integer value: either seconds until expiration, or, 0 to mean never expires. I added this functionality because I wanted to be able to set expiration for status like "lunch" to an hour. Also, the sample's "zoom" profile does not work unless you have :zoom: emoji installed in your Slack, so I changed it to ghost which is a standard emoji that Slack has be default. Hope this helps others and that you can consider merging it @mivok.