[clang-format] Add option to group multiple #include blocks when sorting includes

Summary:
This patch allows grouping multiple #include blocks together and sort all includes as one big block.
Additionally, sorted includes can be regrouped after sorting based on configured categories.

Contributed by @KrzysztofKapusta!

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D40288

llvm-svn: 319024