Support loading nerdctl.toml ($NERDCTL_TOML)

[AkihiroSuda]

See docs/config.md

Replace PR #470

Example

# This is an example of /etc/nerdctl/nerdctl.toml .
# Unrelated to the daemon's /etc/containerd/config.toml .

debug          = false
debug_full     = false
address        = "unix:///run/k3s/containerd/containerd.sock"
namespace      = "k8s.io"
snapshotter    = "stargz"
cgroup_manager = "cgroupfs"

[ningmingxiao]

why use “github.com/pelletier/go-toml” instead of "github.com/spf13/viper“ ?
viper support more format like JSON, TOML, YAML , if user like JSON,YAML config file it is easier to change it.
and go-toml maybe is not as strong as viper.

[AkihiroSuda]

if user like JSON,YAML config file it is easier to change it.

I don't think we need such extensibility.