Add zlog.Field

Because this:

	zlog.Field("site", site.ID)

Is just much nicer than:

	zlog.Fields(zlog.F{"site": site.ID})

zlog.go

@@ -119,6 +119,7 @@ type (
 func Module(m string) Log { return Log{Modules: []string{m}, since: time.Now()} }
 
 func SetDebug(m ...string) Log          { return Log{DebugModules: m} }
+func Field(k string, v interface{}) Log { return Log{Data: F{k: v}} }
 func Fields(f F) Log                    { return Log{Data: f} }
 func Print(v ...interface{})            { Log{}.Print(v...) }
 func Printf(f string, v ...interface{}) { Log{}.Printf(f, v...) }
@@ -150,7 +151,7 @@ func (l Log) Module(m string) Log {
 	return l
 }
 
-// Fields sets the log data.
+// Fields append data to the Log object.
 func (l Log) Fields(f F) Log {
 	if l.Data == nil {
 		l.Data = f
@@ -163,6 +164,11 @@ func (l Log) Fields(f F) Log {
 	return l
 }
 
+// Field sets one data field.
+func (l Log) Field(k string, v interface{}) Log {
+	return l.Fields(F{k: v})
+}
+
 // Print an informational error.
 func (l Log) Print(v ...interface{}) {
 	l.Msg = fmt.Sprint(v...)

zlog_test.go

@@ -28,6 +28,9 @@ func TestLog(t *testing.T) {
 		in   func()
 		want string
 	}{
+		// TODO: get line nr. instead of hard-coding it here.
+		{func() { FieldsLocation().Print("print") }, "INFO: print {location=\"zlog_test.go:32\"}"},
+
 		{func() { Print("w00t") }, "INFO: w00t"},
 		{func() { Printf("w00t %s", "x") }, "INFO: w00t x"},
 		{func() { Error(errors.New("w00t")) }, "ERROR: w00t\n\ttesting.tRunner\n\t\t/fake/testing.go:42"},
@@ -39,6 +42,7 @@ func TestLog(t *testing.T) {
 
 		{func() { Module("test").Fields(F{"k": "v"}).Print("w00t") }, "test: INFO: w00t {k=\"v\"}"},
 		{func() { Module("test").Fields(F{"k": 3}).Print("w00t") }, "test: INFO: w00t {k='\\x03'}"},
+		{func() { Module("test").Fields(F{"k": 3}).Field("k", "1").Print("w00t") }, "test: INFO: w00t {k=\"1\"}"},
 
 		{func() { Module("test").Debug("w00t") }, ""},
 		{func() { SetDebug("xxx").Module("test").Debug("w00t") }, ""},
@@ -61,8 +65,6 @@ func TestLog(t *testing.T) {
 			r, _ := http.NewRequest("PUT", "/path?k=v&a=b", nil)
 			FieldsRequest(r).Error(errors.New("w00t"))
 		}, "ERROR: w00t {http_form=\"\" http_method=\"PUT\" http_url=\"/path?k=v&a=b\"}\n\ttesting.tRunner\n\t\t/fake/testing.go:42"},
-
-		{func() { FieldsLocation().Print("print") }, "INFO: print {location=\"zlog_test.go:65\"}"},
 	}
 
 	for i, tt := range tests {