Support async/await by returning Promise

[zipang]

Callback style APIs are really a thing of the past now that async/await are available in node > 7 !
upndown.convert(html) should return a Promise when no callback is provided !

[zipang]

I provided a Promise implementation by checking if a callback was passed.
Now it's really nice to be able to use await : var markdown = await upndown.convert(html);