Support for basic asciidoc table

The basic simplest [asciidoc table](https://asciidoctor.org/docs/user-manual/#tables) is like the following:

```asciidoc
[cols="2*"]
|===
| Cell in column 1, row 1 | Cell in column 2, row 1
| Cell in column 1, row 2 | Cell in column 2, row 2
| Cell in column 1, row 3 | Cell in column 2, row 3
|===
```

The basic and simplest roles are:

1. The table’s content boundaries are defined by a vertical bar followed by three equal signs (``|===``).
2. The new cell is marked by a vertical bar (``|``).
3. Rows can optionally be separated by any number of blank lines.

Table-mode is working for table cells, but it adds a space between ``|`` and ``===``. Is there any setting to prevent this? If not, I suggest that we should support the basic asciidoc table.