EpicEditor is an embeddable JavaScript Markdown editor with split fullscreen editing, live previewing, automatic draft saving, offline support, and more. For developers, it offers a robust API, can be easily themed, and allows you to swap out the bundled Markdown parser with anything you throw at it.
GFM has support for specifying alignment in tables by using colons in the header row (see https://help.github.com/articles/github-flavored-markdown#tables). Currently github.css overrides this by forcing left alignment for no good reason.