let the author force breaks in a list using trailing comments

This works in clang-format-- the empty comments force the list to be broken as shown:
```
int mymatrix[] = {
1, 2, 3, //
4, 5, 6, //
7, 8, 9 //
};
```
(clang-format aligns the comment markers, so there are indeed 3 spaces after the '9')

In yapf, attempting the same thing causes the whole list to switch to one-element-per-line formatting. Here's another use case, in python, as I'd expect it to stay formatted after yapf:
```
flags = [
'--flag1', val1, #
'--flag2', val2, #
]
```

Current behavior (undesirable, since it's harder to scan and to comment out key-val pairs atomically):
```
flags = [
'--flag1',
val1, #
'--flag2',
val2, #
]
```