Allow buildifier config to exclude paths

[alexeagle]

In rules_nodejs we vendor some code into third_party (like some bits from skylib)

Thus if I try to enable buildifier on bazel CI it says I have to make changes to that code https://buildkite.com/bazel/rules-nodejs-nodejs/builds/9363#e2d87cbb-8310-4f80-a63c-30d849cd6d0c which is not great since then we have diverged from the upstream where we cloned it.

I'd like to say

buildifier:
  exclude: ^third_party/.*

(note, this will further exacerbate the problem of configuring buildifier the same for local dev and for CI. https://github.com/bazelbuild/buildtools/issues/479 is the right fix for that)