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

I 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 +212,9 @@ func TestToDuration(t *testing.T) {
 		assert.Equal(t, v.expected, ToDuration(v.input))
 	}
 }
+
+func TestStringToDate(t *testing.T) {
+	dateString := "2016-12-01 15:05:04"
+	tv, _ := time.Parse("2006-01-02 15:04:05", dateString)
+	assert.Equal(t, ToTime(dateString), tv)
+}

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",
 	})
 }