add "yyyy-MM-dd HH:mm:ss" string format > time.Time

I notice that the Hugo use this project to convert the string to time.Time.
When I migrate to Hugo from Hexo (another static blog generator). The Hugo said :

> page.go:750: Failed to parse date '2016-03-06 15:28:01' in page xxxxx.md

Hope you can merge this Pull request, so that those people who migrate to Hugo from Hexo are easy to use Hugo to generate the static blog.

Thanks.

cast_test.go

@@ -212,3 +212,10 @@ func TestToDuration(t *testing.T) {
 		assert.Equal(t, v.expected, ToDuration(v.input))
 	}
 }
+
+func TestStringToDate(t *testing.T) {
+	d := "2016-03-06 15:28:01"
+	format := "2006-01-02 15:04:05"
+	v, _ := time.Parse(format, d)
+	assert.Equal(t, ToTime(d), v)
+}

caste.go

@@ -513,6 +513,7 @@ func StringToDate(s string) (time.Time, error) {
 		"02 Jan 2006",
 		"2006-01-02 15:04:05 -07:00",
 		"2006-01-02 15:04:05 -0700",
+		"2006-01-02 15:04:05",
 	})
 }