Add --format flag for docker plugin ls

This fix tries to address the enhancement discussed in 28735 to add
`--format` for the output of `docker plugin ls`.

This fix
1. Add `--format` and `--quiet` flags to `docker plugin ls`
2. Convert the current implementation to use `formatter`, consistent with
   other docker list commands.
3. Add `pluginsFormat` for config.json.

Related docs has been updated.

Several unit tests have been added to cover the changes.

This fix is related to 28708 and 28735.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

command/formatter/plugin.go

@@ -0,0 +1,87 @@
+package formatter
+
+import (
+	"strings"
+
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/pkg/stringid"
+	"github.com/docker/docker/pkg/stringutils"
+)
+
+const (
+	defaultPluginTableFormat = "table {{.ID}}\t{{.Name}}\t{{.Description}}\t{{.Enabled}}"
+
+	pluginIDHeader    = "ID"
+	descriptionHeader = "DESCRIPTION"
+	enabledHeader     = "ENABLED"
+)
+
+// NewPluginFormat returns a Format for rendering using a plugin Context
+func NewPluginFormat(source string, quiet bool) Format {
+	switch source {
+	case TableFormatKey:
+		if quiet {
+			return defaultQuietFormat
+		}
+		return defaultPluginTableFormat
+	case RawFormatKey:
+		if quiet {
+			return `plugin_id: {{.ID}}`
+		}
+		return `plugin_id: {{.ID}}\nname: {{.Name}}\ndescription: {{.Description}}\nenabled: {{.Enabled}}\n`
+	}
+	return Format(source)
+}
+
+// PluginWrite writes the context
+func PluginWrite(ctx Context, plugins []*types.Plugin) error {
+	render := func(format func(subContext subContext) error) error {
+		for _, plugin := range plugins {
+			pluginCtx := &pluginContext{trunc: ctx.Trunc, p: *plugin}
+			if err := format(pluginCtx); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	return ctx.Write(&pluginContext{}, render)
+}
+
+type pluginContext struct {
+	HeaderContext
+	trunc bool
+	p     types.Plugin
+}
+
+func (c *pluginContext) MarshalJSON() ([]byte, error) {
+	return marshalJSON(c)
+}
+
+func (c *pluginContext) ID() string {
+	c.AddHeader(pluginIDHeader)
+	if c.trunc {
+		return stringid.TruncateID(c.p.ID)
+	}
+	return c.p.ID
+}
+
+func (c *pluginContext) Name() string {
+	c.AddHeader(nameHeader)
+	return c.p.Name
+}
+
+func (c *pluginContext) Description() string {
+	c.AddHeader(descriptionHeader)
+	desc := strings.Replace(c.p.Config.Description, "\n", "", -1)
+	desc = strings.Replace(desc, "\r", "", -1)
+	if c.trunc {
+		desc = stringutils.Ellipsis(desc, 45)
+	}
+
+	return desc
+}
+
+func (c *pluginContext) Enabled() bool {
+	c.AddHeader(enabledHeader)
+	return c.p.Enabled
+}