Helper function for adding ports

[lucymhdavies]

As per https://github.com/staticli/staticli/issues/2

It is already possible for a (container) cali task to expose ports, but it's kinda awkward to add it:

import "github.com/docker/go-connections/nat"

    task.HostConf.PortBindings = nat.PortMap{
        nat.Port("4000/tcp"): []nat.PortBinding{
            {HostIP: "0.0.0.0", HostPort: "1337"},
        },
    }

Consider a helper function for this, e.g. task.AddPort / task.AddPorts

Consider also having said helper function optionally define flags.

Consider having it such that, if a user specified port is not available, it will pick one at random, or just refuse to start (depending on user/developer preference?)