Support multi-row and row span table syntax

[a60814billy]

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This PR support two new table syntax, one is multi-row syntax that use \ to merge multiple row into one row. another is row-span syntax that can merge multiple column into one colume (like html rowspan attribute)

1. multi-row syntax example

|   Markdown   | Rendered HTML |
|--------------|---------------|
|    *Italic*  | *Italic*      | \
|              |               |
|    - Item 1  | - Item 1      | \
|    - Item 2  | - Item 2      |
|    ```python | ```python       \
|    .1 + .2   | .1 + .2         \
|    ```       | ```           |

2. row span syntax

Stage | Direct Products | ATP Yields
----: | --------------: | ---------:
Glycolysis | 2 ATP ||
^^ | 2 NADH | 3--5 ATP |
Pyruvaye oxidation | 2 NADH | 5 ATP |
Citric acid cycle | 2 ATP ||
^^ | 6 NADH | 15 ATP |
^^ | 2 FADH2 | 3 ATP |
**30--32** ATP |||
[Net ATP yields per hexose]

[Yukaii]

Nice! But found that our existing table auto format would break this syntax

[a60814billy]

it's a known issue, maybe we can add preference options to turn off table auto formatting.