Boost.Tuple: implement set_open(const char *)

[Vladimir Rutsky <altsysrq@…>]

Current implementation of Boost.Tuple IO manipulators allows define only single character for start of tuple, delimiter of items and for end of tuple.

1. Sometimes it is convenient to have no character before and after tuple data.
2. Sometimes it is convenient to wrap tuple in more that single character.

For example output tuple as HTML table row: with open string "<tr><td>", delimiter "</td><td>" and end string "</td></tr>", so (1 2 3) will be outputted as correct HTML table row "<tr><td>1</td><td>2</td><td>3</td></tr>".