Add trailing comma support for multiline exports (#168)
Add trailing comma support for multiline exports (#168)

```
echo "export { aaaaaaaa, bbbbbb, cccccc, dddddddd, eeeeeee, fffffffff, ggggggggg } from 'vjeux';" | ./bin/prettier.js --stdin --trailing-comma
export {
  aaaaaaaa,
  bbbbbb,
  cccccc,
  dddddddd,
  eeeeeee,
  fffffffff,
  ggggggggg,
} from "vjeux";
```
lydell/prettier,Skywalker13/prettier-space-parenthesis,arijs/prettier-miscellaneous,despairblue/prettier,lydell/prettier,lydell/prettier,yamafaktory/prettier,prettier/prettier,jlongster/prettier,prettier/prettier,azz/prettier,rattrayalex/prettier,despairblue/prettier,prettier/prettier,yamafaktory/prettier,rattrayalex/prettier,arijs/prettier-miscellaneous,azz/prettier,Skywalker13/prettier-space-parenthesis,danharper/prettier,azz/prettier,rattrayalex/prettier,existentialism/prettier,Pajn/prettier,existentialism/prettier,prettier/prettier,arijs/prettier-with-tabs,existentialism/prettier,lydell/prettier,azz/prettier,Skywalker13/prettier-space-parenthesis,Pajn/prettier,arijs/prettier-miscellaneous,Skywalker13/prettier-space-parenthesis,arijs/prettier-miscellaneous,existentialism/prettier,vjeux/prettier,arijs/prettier-with-tabs