Support multi-row and row span table syntax

[a60814billy]

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.