Support Markdown table alignment

I've added the config to generate the correct HTML to use a CSS class. Also added a test case to verify.

Still need to add the library and link to the output HTML.

Added a library with CSS classes for left, centre, and right table cells.

It seems a waste to always load this, since it will only be required by a minority of markdown that makes use of tables.

To prevent this, I have added a regex check for table cells with relevant classes, in the converted text, and only load the library if needed.

I check for the presence of the CSS file, and table cells in FilterTest.

Ready for review.

Looks good, @lostcarpark!

I especially like the bit about only loading the library when the proper alignment syntax is present! 🤘🏼

Also, thanks for consolidating some of the test strings into constants.

Merged.

thank you!
-mike