Buildifier should reflow single-item lists

Right now buildifier leaves single-item lists alone. That is, if it sees something like:
```
srcs = [
"foo",
],
```
or like:
```
srcs = ["foo"],
```
it will leave them both as-is. But I would like to force it to always use one or the other for consistency (in my case, I would like to force single-item lists onto one line). Is this already possible and I just can't figure out how?
Thanks!

Start by checking buildifier's existing formatting options and how it handles the two single-item list forms shown in the issue. Determine whether a setting can consistently select the one-line form, and verify that the chosen behavior is covered for both examples.

Written by the indexing model from the issue text.