Remove Markdown parsing

[michaelrog]

• Currently, the markdown and md filters are deprecated, because Craft provides them natively, and we don't want Wordsmith to conflict. (Add encode parameter to markdown & md filters #57)
• Parsedown Extra is also antiquated; we need to ditch it. (Dependency issue causing CI to fail #59)

Given that Craft's native markdown filter supports more flavors, I think it's safe to remove Markdown parsing from Wordsmith altogether.

However, I like that Wordsmith, string methods are exposed as both Twig functions and Twig filters. So maybe we can keep the md() function in place (but have it pass through to Craft's parser), and let Craft own the |md filter.