Support for orgtbl-mode style tables in markdown

[Berengal]

orgtbl-mode is a minor mode for Emacs that makes it easy to create simple tables in plain text files. It comes with org-mode, which is built-in, and it's therefore very convenient to use when creating simple tables.

Unfortunately, the syntax doesn't match the pandoc markdown table syntax. The orgtbl-mode table syntax is designed for plain text documents, and therefore fit the markdown plain text philosophy very well. I therefore wish this could be implemented as an alternative table syntax.

Syntax example:

| Header | Second header |
|--------+---------------|
| cell   | Another cell  |
|        | <- empty cell |

[dfc]

Take a look at #260

[jgm]

The development version of pandoc adds limited support for orgtbl-mode style tables.
See the README under 'Pipe tables' for details.