clang-format: [JS] Introduce JavaScriptWrapImports option.

Summary:
When turned on, clang-format wraps JavaScript imports (and importing exports),
instead of forcing the entire import statement onto one line.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D21273

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272558 91177308-0d34-0410-b5e6-96231b3b80d8