Support for spanning columns and rows?

Hi @dhruvasagar ,

It's wonderful to have your plugin to VIM. I'm trying to work on the spanning multiple rows and columns via vim-table-mode. So far, I haven't been able to achieve that. I tried to look for online help but to no avail. Would you mind sharing how I can do that?

This is what I want to achieve:

```
TABLE 1:
+------------+------------+-----------+
| Header 1 | Header 2 | Header 3 |
+============+============+===========+
| body row 1 | column 2 | column 3 |
+------------+------------+-----------+
| body row 2 | Cells may span columns.|
+------------+------------+-----------+
| body row 3 | Cells may | - Cells |
+------------+ span rows. | - contain |
| body row 4 | | - blocks. |
+------------+------------+-----------+
```

And yet this is what I get
```
TABLE 2:
+------------+-------------------------+----------+
| Header 1 | Header 2 | Header 3 |
+============+=========================+==========+
| body row 1 | column 2 | column 3 |
+------------+-------------------------+----------+
| body row 2 | Cells may span columns. |
+------------+-------------------------+----------+
| body row 3 | Cells may | - Cells |
+------------+ span rows. | - contain |
| body row 4 | | - blocks. |
+------------+--+-----------+
```

I copied and pasted the above TABLE 1 into vim with Table Mode Enabled, it rendered well. Only until I moved the cursor inside the table, did it start to automatically rearranged every cells till it became TABLE 2.

I really appreciate your work and hope you can help me with this issue.

Regards,
JP