Improvements to allow Markdown based slides

[JonTheNiceGuy]

• Remove "wp_texturize" by default
• Add a new markdown field per-slide, separate from the data attribute field
• Markdown should be edited in a textarea, not a wp_editor

[JonTheNiceGuy]

One outstanding item: I need to figure out how to stop wpautop from being triggered when using a Markdown slide, as that's causing some issues in rendering.