Format timestring using string-defined format

Tests: https://github.com/deadtrickster/local-time/blob/b9d17aadaaa2eb0fd8f7ed529dab3e8e7658088e/test/formatting.lisp#L106

```lisp
 (frob
     "4:5:6" "h:M:s"
     "20141103" "yyyymmdd"
     "Mon Nov 03 2014 04:05:06" "ddd mmm dd yyyy HH:MM:ss"
     "11/03/14" "m/dd/yy"
     "Nov 3, 2014" "mmm d, yyyy"
     "November 3, 2014" "mmmm d, yyyy"
     "Monday, November 3, 2014" "dddd, mmmm d, yyyy"
     "4:05 AM" "h:MM TT"
     "4:05:06 AM" "h:MM:ss TT"
     "4:05:06 AM PST" "h:MM:ss TT Z"
     "2014-11-03" "yyyy-mm-dd"
     "04:05:06" "HH:MM:ss"
     "2014-11-03T04:05:06" "yyyy-mm-dd\\THH:MM:ss"
     "Mon, 03 Nov 2014 04:05:06 -0800" "ddd, dd mmm yyyy hh:MM:ss O" ;;rfc1123
     "2014-11-03T04:05:06.416462-08:00" "yyyy-mm-dd\\THH:MM:ss.uuuuuuo" ;;iso8601
     "it is the 3rd day" "i\\t i\\s \\t\\he S \\da\\y"
     "Printed on 11/03/2014 at 04:05AM" "Prin\\te\\d \\on mm/dd/yyyy a\\t hh:MMTT"))))
```

PS this test
```lisp
"Thu, 05 Jun 2008 04:03:02 UTC"
       (format-timestring nil test-timestamp :format +rfc-1123-format+)
``` 
fails..