Add more functionality to the CLI editor

[danny-lloyd]

For example add-entry currently adds an incident with one update at the same time as the main incident.

It would be good to have e.g. add-future-entry that creates an update with the current date but asks for the date of the future incident. Then you could add planned maintenance windows.

Another might be add-update which lets you pick from the current incidents and lets you add an update to them.

Another might be clean-up which removes old/expired incidents (see #17). This would keep the status page from being too long.