Allow for selection of Markdown engine; default to markdown package, if available

[rich-iannone]

This PR allows for a choice of the Markdown renderer in the fmt_markdown() and vec_fmt_markdown() functions; the renderer can either be from the markdown package (new default) or the commonmark package (as before). However, the choice only applies to HTML and LaTeX outputs. Furthermore, the markdown package is a soft dependency. If it is available and the choice is "auto", gt will choose markdown, otherwise commonmark.

Fixes: #725