Support opening on untitled editors

* Supports opening on untitled buffers
* Renders as you type (no more saving required)
* Adds a list of grammars to the config that the command should be enabled for. The setting is `markdown-preview.grammars` and similar to how the spell-check package does it

![markdown-preview](https://f.cloud.github.com/assets/671378/2336626/0f23ccb2-a492-11e3-816a-d663a5e1f986.gif)

Closes #15 
Closes #19