Added config options for formatting multiple imports

[rbtying]

This enables rustfmt to format

use a::{
    b,
    c,
}

or

use a::{b,c}

as desired.

[jimmycuadra]

Is this PR necessary to prevent rustfmt from turning this:

use rusoto_ecr::{
    DescribeImagesRequest,
    DescribeRepositoriesRequest,
    Ecr,
    EcrClient,
    ImageDetailList,
    RepositoryList,
};

into this?

use rusoto_ecr::{DescribeImagesRequest, DescribeRepositoriesRequest, Ecr, EcrClient,
                 ImageDetailList, RepositoryList};

If so, could someone please review and merge this?

[nrc]

I'm not sure how this missed a review, very sorry @rbtying . I think this is probably subsumed by #1760 and/or #1785