Added AlwaysBreakBeforeMultilineStrings option.

Summary:
Always breaking before multiline strings can help format complex
expressions containing multiline strings more consistently, and avoid consuming
too much horizontal space.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1097

llvm-svn: 185622