Clang-format: allow -style="{yaml/json}" on command line

Summary: + improved handling of default style and predefined styles.

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D813

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@182205 91177308-0d34-0410-b5e6-96231b3b80d8