clang-format: Add option for the offset of constructor initializers.

Some coding styles use a different indent for constructor initializers.

Patch by Klemens Baum. Thank you.
Review: http://llvm-reviews.chandlerc.com/D1360

Post review changes: Changed data type to unsigned as a negative indent
width does not make sense and added test for configuration parsing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188260 91177308-0d34-0410-b5e6-96231b3b80d8