clang-format: Allow customizing the penalty for breaking assignment

Summary:
Add option to customize the penalty for breaking assignment

This allows increasing the priority of the assignment, to prefer spliting
an operation instead of splitting the assignment, e.g. :

  int a = bbbbbbbbbbbbbbbb +
          cccccccccccccccc;

Reviewers: krasimir, djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D32477

llvm-svn: 303534