Screenshot needs to be deleted from filesystem

[Glench]

Right now the only place the screenshot is being deleted is after a Twitter post:

twitter.post('statuses/update', params, function(err) {
    if (err) {
        console.log(err)
    }
})
fs.unlink(screenshot)

The screenshot needs to be deleted in the following cases: if the user only is posting to Mastodon and not Twitter, if the screenshot was not uploaded to Twitter successfully (the callback returns so the fs.unlink code is never hit).

I used async for the screenshot code before because it simplified the coordination of all these async network requests a lot.

[edsu]

Thanks for catching this. As part of #159 I'm going to reintroduce your async/await code which should make handling this easier. I'll leave this ticket open as a reminder to double check the screenshot isn't left sitting around for this case where Mastodon is configured and Twitter is not.