clang-format: Add option to suppress operator alignment.

With alignment:
  int aaaaaa = aa
               + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
                     * cccccccccccccccccccccccccccccccc;

Without alignment:
  int aaaaaa = aa
      + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
          * cccccccccccccccccccccccccccccccc;

This fixes llvm.org/PR21666.