Support Tibbe-style alignment of 'where' clauses

[int3]

I've implemented the ability to 'dedent' the where clause by a half-shift-width, as described in these style guides. If you're interested, I'll put it under an option setting and send in a PR. The commit responsible is at int3@cec81d7b8cc0bfddf4958211f83bfdbb39e2ce62