WTF-986 User-definable DigitalOcean columns

[senorprogrammer]

This PR allows the user to define which droplet property columns to display, by defining them as a list in their config. Falls back to a default set of columns if none are explicitly defined.

The new settings look like:

digitalocean:
    columns:
        - Name
        - Status
        - Vcpus
        - Disk
        - Memory
        - Region.Slug

In this case it would display those six columns. If no config settings are defined, the default set is used:

var defaultColumns = []interface{}{
	"Name",
	"Status",
	"Region.Slug",
}

Valid configuration values are any publicly-accessible properties on a DigitalOcean Droplet, Image, Region, or Size.

See https://github.com/digitalocean/godo for more details about those.

Closes #986.