cli: add system command and subcmds to interact with system API.

The system command includes gc and reconcile-summaries subcommands
which covers all currently available system API calls. The help
information is largely pulled from the current Nomad website API
documentation.

command/system_test.go

@@ -0,0 +1,12 @@
+package command
+
+import (
+	"testing"
+
+	"github.com/mitchellh/cli"
+)
+
+func TestSystemCommand_Implements(t *testing.T) {
+	t.Parallel()
+	var _ cli.Command = &SystemCommand{}
+}