Add support for tables

[sgjesse]

Currently support for tables is missing.

Running the following code:

import 'package:markdown/markdown.dart';

var table =
r'''
First Header  | Second Header
------------- | -------------
Content Cell  | Content Cell
Content Cell  | Content Cell
''';

main() {
  print(markdownToHtml(table));
}

prints a paragraph tag with the text.

<p>First Header  | Second Header
------------- | -------------
Content Cell  | Content Cell
Content Cell  | Content Cell</p>

It should print something like this:

<table>
<thead>
<tr><th>First Header</th><th>Second Header</th></tr>
</thead>
<tbody>
<tr><td>Content Cell</td><td>Content Cell</td></tr>
<tr><td>Content Cell</td><td>Content Cell</td></tr>
</tbody>
</table>

[munificent]

It's worth noting that tables are a GitHub extension and aren't part of standard Markdown or CommonMark. Also, you can always use inline HTML for a table.