Filter out bindgen's `C-assigned` issues; they're not available

[fitzgen]

They're already assigned.

Perhaps this functionality should be generalized somehow.

[budziq]

Yep cookbooks WIP label should be filtered out too.

I've done a little digging. It seams that negative label filters are available in the github search/issues API and not in the basic repos/{}/issues API used by findwork.

The query would be more involved (note the +-label:wip)

curl 'https://api.github.com/search/issues?q=repo:"rust-lang-nursery/rust-cookbook"+is:open+is:issue+label:easy+label:example+-label:wip'

and the output json array is returned within a dict but the array itself has the same format.

[budziq]

Darn It looks like the search/issues endpoint does not allow for filtering by milestones. I guess that the best course of action would be to query second time if there are any any negative labels and filter these by hand in the backend.