Add --since argument to docker logs cmd

Added --since argument to `docker logs` command. Accept unix
timestamps and shows logs only created after the specified date.

Default value is 0 and passing default value or not specifying
the value in the request causes parameter to be ignored (behavior
prior to this change).

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

timeutils/utils.go

@@ -0,0 +1,22 @@
+package timeutils
+
+import (
+	"strconv"
+	"time"
+)
+
+// GetTimestamp tries to parse given string as RFC3339 time
+// or Unix timestamp, if successful returns a Unix timestamp
+// as string otherwise returns value back.
+func GetTimestamp(value string) string {
+	format := RFC3339NanoFixed
+	loc := time.FixedZone(time.Now().Zone())
+	if len(value) < len(format) {
+		format = format[:len(value)]
+	}
+	t, err := time.ParseInLocation(format, value, loc)
+	if err != nil {
+		return value
+	}
+	return strconv.FormatInt(t.Unix(), 10)
+}