lua - don't align='center' floats of type Listing

[cscheid]

Consider this minimal example:

```{.python filename="run_experiments.py" #lst-1 lst-cap="Corpus Loading/Generation Logic"}
line 1
line 2
```

That currently produces centered output:

This comes from the fact that the default alignment for floats is center, and our CSS centers all paragraphs and divs.

This bug is something that we should fix by:

• redesigning the HTML output of our floats
• changing our CSS to not use text-align: center

Those changes are not something I want to do late in 1.5, so we will instead make a small workaround that sets the alignment for Listing floats to always be left.