Adds nomad monitor command

Adds nomad monitor command. Like consul monitor, this command allows you
to stream logs from a nomad agent in real time with a a specified log
level

add endpoint tests

Upgrade go-hclog to latest version

The current version of go-hclog pads log prefixes to equal lengths
so info becomes [INFO ] and debug becomes [DEBUG]. This breaks
hashicorp/logutils/level.go Check function. Upgrading to the latest
version removes this padding and fixes log filtering that uses logutils
Check

command/agent_monitor_test.go

@@ -0,0 +1,34 @@
+package command
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/mitchellh/cli"
+)
+
+func TestMonitorCommand_Implements(t *testing.T) {
+	t.Parallel()
+	var _ cli.Command = &MonitorCommand{}
+}
+
+func TestMonitorCommand_Fails(t *testing.T) {
+	t.Parallel()
+
+	ui := new(cli.MockUi)
+	cmd := &MonitorCommand{Meta: Meta{Ui: ui}}
+
+	// Fails on misuse
+	if code := cmd.Run([]string{"some", "bad", "args"}); code != 1 {
+		t.Fatalf("exepected exit code 1, got: %d", code)
+	}
+	if out := ui.ErrorWriter.String(); !strings.Contains(out, commandErrorText(cmd)) {
+		t.Fatalf("expected help output, got: %s", out)
+	}
+
+	ui.ErrorWriter.Reset()
+
+	if code := cmd.Run([]string{"-address=nope"}); code != 1 {
+		t.Fatalf("exepected exit code 1, got: %d", code)
+	}
+}