tweaked pretty print table code

Tweaked the code so that there is now a "strict" mode. In this mode it only prints a row if all elements in that row are not empty.

An example in non-strict mode:

sa: load foaf-example-in-sw.sw
sa: table[name,email,works-for] "" |list>
+------------------------+----------------------------------+------------------------+
| name                   | email                            | works-for              |
+------------------------+----------------------------------+------------------------+
| Dan                    | email: danbri@w3.org             | organisation: ILRT     |
| Libby                  | email: libby.miller@bris.ac.uk   | organisation: ILRT     |
| Craig                  | email: craig@netgates.co.uk      | organisation: Netgates |
| Liz                    |                                  | organisation: Netgates |
| Kathleen               |                                  | organisation: Netgates |
| Damian                 |                                  |                        |
| Martin                 | email: m.l.poulter@bristol.ac.uk |                        |
| organisation: ILRT     |                                  |                        |
| organisation: Netgates |                                  |                        |
+------------------------+----------------------------------+------------------------+

Same example, but in strict mode:

sa: strict-table[name,email,works-for] "" |list>
+------------------------+----------------------------------+------------------------+
| name                   | email                            | works-for              |
+------------------------+----------------------------------+------------------------+
| Dan                    | email: danbri@w3.org             | organisation: ILRT     |
| Libby                  | email: libby.miller@bris.ac.uk   | organisation: ILRT     |
| Craig                  | email: craig@netgates.co.uk      | organisation: Netgates |
+------------------------+----------------------------------+------------------------+

And that's it. Heaps more to come!