feat: add no_background property

[mfontanini]

This renames the no_margin attribute added in #363 to no_background and makes it do everything no_margin did but also prevents presenterm from adding a background to the snippet. This currently doesn't play well with exec_replace or exec meaning the snippet execution output still gets background. I will address this separately as there's other things I want to get to but I want to close this issue.

The following

```text +no_background
.__    .__
|  |__ |__|   _____   ____   _____
|  |  \|  |  /     \ /  _ \ /     \
|   Y  \  | |  Y Y  (  <_> )  Y Y  \
|___|  /__| |__|_|  /\____/|__|_|  /
     \/           \/             \/
```

Currently renders like this after this change:

Fixes #349

[chris-t-jansen]

This worked perfectly for me, thank you so much! Exactly what I needed.