Add support for org-table syntax

This just adds support for alternatively using a plus (`+`) as an intersection character instead of requiring pipes (`|`) when drawing the header dashes of a table. The emacs org-mode table automatically manages ascii tables, but requires the use of pluses for line intersections. I think it would be great if markdown tables could support this syntax as well.

Here's an example of a table in org-table syntax. The only difference from the already-supported markdown table syntax is the use of pluses instead of pipes in the header dash line.

```
| Column A | Column B | Column C |
|----------+----------+----------|
| Cell A1  | Cell B1  | Cell C1  |
| Cell A2  | Cell B2  | Cell C2  |
| Cell A3  | Cell B3  | Cell C3  |
```

Which would produce this:

| Column A | Column B | Column C |
|----------|----------|----------|
| Cell A1  | Cell B1  | Cell C1  |
| Cell A2  | Cell B2  | Cell C2  |
| Cell A3  | Cell B3  | Cell C3  |