Allow content to be written using markdown formatting

[dmcassel]

The current formatting needs to continue to work, but providing a Markdown alternative would be really nice. Side-by-side text and preview would be sweet. Might be tricky given how much XHTML processing goes on.

[tpiros]

@dmcassel This should do the job. I believe DMC also uses jQuery so it should be a seemless integration: https://github.com/timsayshey/Ghost-Markdown-Editor