Feature: Support pandoc grid tables

[jakewvincent]

Is your feature request related to a problem? Please describe.

Currently, mkdnflow's table formatting only supports pipe tables. Pandoc markdown supports additional table formats that would be useful to handle.

Describe the solution you'd like

Support for pandoc grid tables, which look like:

+---------------+---------------+--------------------+
| Fruit         | Price         | Advantages         |
+===============+===============+====================+
| Bananas       | $1.34         | - built-in wrapper |
|               |               | - bright color     |
+---------------+---------------+--------------------+
| Oranges       | $2.10         | - cures scurvy     |
|               |               | - tasty            |
+---------------+---------------+--------------------+

Grid tables allow for more complex layouts including multi-line cells and block elements within cells.

Additional context

Spun off from this comment on #243 by @iGormilhit.