Preserve trailing commas

[webdevred]

Summary:
Hpack removes trailing commas from multi-line lists, e.g., build-depends and extra-source-files. Related #617

Example:

Before:

  vector >=0.13,

After hpack:

  vector >=0.13

Expected behavior:
Keep trailing commas when they exist in the original file.

Reason:
Trailing commas reduce diff noise when adding/removing elements in a list.

[sol]

This is basically:

build-depends:
    foo
  , bar
  , baz

vs

build-depends:
  foo,
  bar,
  baz,

The second option is strictly better than the first. However, I don't know since when Cabal has been supporting the redundant trailing comma for the last item.

I need to know:

1. Which version of Cabal introduced support for the redundant trailing comma?

Once we have that information, this is easy to do. Specifically, I think we want to:

1. Default to trailing commas, provided the target cabal-version supports the redundant comma for the last list item.
2. If there is an existing .cabal file then we still want to sniff the comma style.