Improvements to clang-format integrations.

This adds an emacs editor integration (thanks to Ami Fischman). Also
pulls out the style into a variable for the vi integration and just
uses clang-formats defaults style in clang-format-diff.py.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179098 91177308-0d34-0410-b5e6-96231b3b80d8