Add support for org-table syntax

[rmm5t]

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

[robin850]

Seems good to me! Thanks for this contribution. Good one!

[rmm5t]

Sweet. Thanks @robin850! Can't wait to see this supported in GitHub READMEs.

[mattr-]

Sorry, but GitHub doesn't use RedCarpet anymore, so I don't believe this
will show up there unless they pull it in manually somehow.

[rmm5t]

Ohs Noes! 😞

@mattr- Do you know what they're currently using so I can go submit a patch there too?

[mattr-]

See #212 (comment)

It's not open source, AFAIK

[rmm5t]

@vmg Who do I need to convince at GitHub to to add this small variant of the markdown table syntax to the GitHub markdown codebase so tables in READMEs are easier to maintain?

[FSX]

What about Sundown?

[mattr-]

Still in use by redcarpet, but deprecated according to the README

[FSX]

Well, ok... I guess I'll just fork Sundown and backport changes in Redcarpet to my fork.