feat: add +image to code blocks to consume their output as an image

[mfontanini]

This adds a new +image attribute to code blocks. This inherits +exec_replace (meaning it has the same semantics and requires being enabled with the same parameters) but it assumes the output of the executed block is an image and renders it as such.

This means a presentation like the following one:

hi
----

```bash +image
curl -s -L -o - 'https://github.com/mfontanini/presenterm/blob/master/examples/doge.png?raw=true'
```

Renders like this

For this to work, the entire output of the code snippet must be an image written to stdout.

An application of this feature could be to have text to image conversions to create titles on the fly. Hopefully this opens up the door for more creative ideas users will definitely have.