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.

llvm-svn: 188260