Add option to disable auto previewing. Allow preview on-demand via new command

This is for issue #29. I ended up implementing it a little differently. Two things to note:
1. Instead of toggling preview, I simply added command to start the preview. When the buffer exists, the preview stops automatically
2. I'm not particularly fond of the way the plugin handles multiple markdown buffers with the push/pop logic, so I opted to keep the manual option simple where it only works on the currently edited buffer