commands: Make the --poll flag a duration

So you can do:

```
hugo server --poll 700ms
```

See #8720

common/types/convert.go

@@ -18,10 +18,30 @@ import (
 	"fmt"
 	"html/template"
 	"reflect"
+	"time"
 
 	"github.com/spf13/cast"
 )
 
+// ToDuration converts v to time.Duration.
+// See ToDurationE if you need to handle errors.
+func ToDuration(v interface{}) time.Duration {
+	d, _ := ToDurationE(v)
+	return d
+}
+
+// ToDurationE converts v to time.Duration.
+func ToDurationE(v interface{}) (time.Duration, error) {
+	if n := cast.ToInt(v); n > 0 {
+		return time.Duration(n) * time.Millisecond, nil
+	}
+	d, err := time.ParseDuration(cast.ToString(v))
+	if err != nil {
+		return 0, fmt.Errorf("cannot convert %v to time.Duration", v)
+	}
+	return d, nil
+}
+
 // ToStringSlicePreserveString is the same as ToStringSlicePreserveStringE,
 // but it never fails.
 func ToStringSlicePreserveString(v interface{}) []string {

common/types/convert_test.go

@@ -16,6 +16,7 @@ package types
 import (
 	"encoding/json"
 	"testing"
+	"time"
 
 	qt "github.com/frankban/quicktest"
 )
@@ -36,3 +37,13 @@ func TestToString(t *testing.T) {
 	c.Assert(ToString([]byte("Hugo")), qt.Equals, "Hugo")
 	c.Assert(ToString(json.RawMessage("Hugo")), qt.Equals, "Hugo")
 }
+
+func TestToDuration(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(ToDuration("200ms"), qt.Equals, 200*time.Millisecond)
+	c.Assert(ToDuration("200"), qt.Equals, 200*time.Millisecond)
+	c.Assert(ToDuration("4m"), qt.Equals, 4*time.Minute)
+	c.Assert(ToDuration("asdfadf"), qt.Equals, time.Duration(0))
+
+}