Support escaped `> table cell values (selectively avoid spanning)

It should be possible to create table cells with `>` as their sole content, e.g.:

| VIM Shortcut | Description |
|----------|-------------|
| >       | Indent      |

Currenty, `>` always leads to column spanning. With this patch, the spanning can be suppressed by escaping the value with a backtick:  <tt>`></tt>