clang-format support for multiple input files.

Summary:
Added support for multiple input files, that can be used both with and
without in-place edit (-i) option. Added checks for -offset and -length options:
don't allow them on multiple files, check that they don't fall outside input
file, made both options unsigned, so that there's no need to check for negative
values. Removed "-1 can be used for end-of-file" from -length description.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D719