Add CSV parser For smart cell

[josevalim]

The cell Will be an editor but it will also support drag and dropping a file. We will use NimbleCSV for parsing. Although we will most likely need to add a struct that will be returned by the streaming API.

There is an open question all what to do we file uploads. We will write it to disk first? We will load the contents into the editor? We will persist its contents in the livemd file?

[josevalim]

Actually, we should likely use Explorer for parsing instead of NimbleCSV.