Support status expiration dates

[pschmitt]

This adds support for the status_expiration parameter.

It value is set via the -e|--expire flag and can either be:

• an arbitrary UNIX timestamp (default)
• if prefixed with +: a number of seconds that will get added to the current time (eg: +300 for 5 minutes, +3600 for 1 hour)

https://api.slack.com/docs/presence-and-status#user-presence-and-status__custom-status__reading-statuses

[pschmitt]

Oh. I should probably have checked the open PRs before submitting a new one.

#98 is very similar.