Sync-scrolling + tables support

The approach taken for sync-scrolling is simple and far from perfect, but the result is still way better than the previously implemented strategy, which was dubbed "auto-scrolling to the bottom of the preview panel". Expect sync-scrolling to be revamped for perfect synchronization in the future.

This release also boasts a new Markdown converter, which is way faster than the older, and supports GFM tables among other things.