Multi-row Header Support

[wnm3]

Would you please consider supporting multiple header rows when rendering tables?

An example would be a table like:

|             | Mars     | Mars   | Venus    | Venus  |
|             | Produced | Sold   | Produced | Sold   |
| ----------- | -------- | ------ | -------- | ------ |
| Teddy Bears | 50,000   | 30,000 | 100,000  | 80,000 |
| Board Games | 10,000   | 5,000  | 12,000   | 9,000  |

which renders like this in Fletcher Penney's MultiMarkdown Viewer:

But renders as follows in github: | | Mars | Mars | Venus | Venus |

	Produced	Sold	Produced	Sold
Teddy Bears	50,000	30,000	100,000	80,000
Board Games	10,000	5,000	12,000	9,000