printf

A wrapper around fprintf and sprintf with the following features:
* Print to multiple file pointers.
* Append directly to files (given file name).
* Return the output as a string.
* Flexible support for printing lists.

Example:
>> s = printf({1,'foo.log'}, 'len(c)=%d, c=[%s].\n', length(c), {'%g', ' ', c});
len(c)=3, c=[3.14 1 15].
>> s2 = printf('<%s>', {[1 3 4 5]})

s2 =

<1, 3, 4, 5>

>>

Also append the printed text to file foo.log, and return it (incl the line break) to s.