setup: help with protected branch

When using `toReleaseBranch` strategy, there can be a possibility that the working branch is protected. It means after release, when Ship.js tries to merge the release branch back to the working branch, it will fail to push the merge commit. In that case, there's nothing much that can be done programmatically.

So if we can check via GitHub API

(1) if this branch is protected
(2) and this branch allows force push,

then we can enable `forcePushBranches` config by default.

Start with the toReleaseBranch strategy and the GitHub API integration. Check whether the working branch is protected and whether force pushes are allowed, then enable forcePushBranches by default only when appropriate; done means releases can handle protected working branches without a failed merge push.

Written by the indexing model from the issue text.