feat(comp): Add descriptions for plugin completion

Ref: HIP 0008

When completing a plugin name, extra information will be shown for
shells that support completions (fish, zsh).  For example:

$ helm plugin uninstall <TAB>
2to3             -- migrate and cleanup Helm v2 configuration and releases in-place to Helm v3
diff             -- Preview helm upgrade changes as a diff
fullstatus       -- provide status of resources part of the release
github           -- Install or upgrade Helm charts from GitHub repos

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

cmd/helm/plugin_list.go

@@ -83,7 +83,7 @@ func compListPlugins(toComplete string, ignoredPluginNames []string) []string {
 		filteredPlugins := filterPlugins(plugins, ignoredPluginNames)
 		for _, p := range filteredPlugins {
 			if strings.HasPrefix(p.Metadata.Name, toComplete) {
-				pNames = append(pNames, p.Metadata.Name)
+				pNames = append(pNames, fmt.Sprintf("%s\t%s", p.Metadata.Name, p.Metadata.Usage))
 			}
 		}
 	}

cmd/helm/testdata/output/plugin_list_comp.txt

@@ -1,7 +1,7 @@
-args
-echo
-env
-exitwith
-fullenv
+args	echo args
+echo	echo stuff
+env	env stuff
+exitwith	exitwith code
+fullenv	show env vars
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp

cmd/helm/testdata/output/plugin_repeat_comp.txt

@@ -1,6 +1,6 @@
-echo
-env
-exitwith
-fullenv
+echo	echo stuff
+env	env stuff
+exitwith	exitwith code
+fullenv	show env vars
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp