clang-format: New option SpacesInSquareBrackets.

Before:
  int a[5];
  a[3] += 42;

After:
  int a[ 5 ];
  a[ 3 ] += 42;

Fixes LLVM bug #17887 (http://llvm.org/bugs/show_bug.cgi?id=17887).

Patch by Marek Kurdej, thank you!