Cells merging

[rlalik]

This is feature I think many have waited long: #65

I needed to rewrite a bit internal structures. I see some room for further optimization but in the separate PR as it is not related to my code.

Code example:

  table.add_row(Row_t{"A", "", "", ""});
  table.add_row(Row_t{"B", Merge{}});
  table.add_row(Row_t{"C", Merge{1}});
  table.add_row(Row_t{"D", Merge{2}});
  table.add_row(Row_t{"", "F", Merge{1}});
  table.add_row(Row_t{"", "G", Merge{2}});

  table[0][0].format().width(5);
  table[0][1].format().width(5);
  table[0][2].format().width(5);
  table[0][3].format().width(5);

  table[1][0].format().font_align(FontAlign::center);
  table[2][0].format().font_align(FontAlign::center);
  table[3][0].format().font_align(FontAlign::center);
  table[4][1].format().font_align(FontAlign::center);
  table[5][1].format().font_align(FontAlign::center);

Result:

+-----+-----+-----+-----+
| A   |     |     |     |
+-----------+-----+-----+
|     B     |     |     |
+-----------+-----+-----+
|     C     |     |     |
+-----------------+-----+
|        D        |     |
+-----+-----------+-----+
|     |     F     |     |
+-----+-----------------+
|     |        G        |
+-----+-----------------+

[rlalik]

I am also thinking about another improvement, that instead of:

table.add_row(Row_t{"C", Merge{1}});

one can also write:

table.add_row(Row_t{Merge{"C", 2}});

What do you think?