Added an option to specify fallback style.

Summary:
Added -fallback-style option. Changed clang-format to stop searching
for .clang-format when an invalid file is found.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2292

llvm-svn: 196108