Consider supporting trailing commas within a list

Sometimes it's convenient to be able to comment out the last line in a list, which leads to parse errors due to the trailing comma. For example:

```
foo = [
  "bar",
  "baz",
  "buz"
]
```

Is valid, but if the last line is commented out, e.g.:

```
foo = [
  "bar",
  "baz",
  # "buz"
]
```

Then a parse error occurs due to the trailing comma.
