Adds undocumented/experimental “master issue” feature.

Setting `config.masterIssue = true` will result in Renovate opening and maintaining an issue that contains a list of all PRs both pending and open, and allowing some control over them (e.g. bypass schedule, force retry, etc).

Setting `config.masterIssueApproval` in addition will mean that branches are not created automatically and instead await approval in that master issue.

Closes #2595

Instead of aborting PR when lock file errors happen, raise the PR with PR comment instead (like with npm/yarn).

Closes #2587

If PR title starts with “rebase!” then PR will be rebased.

Closes #2536

Do no lock file maintenance work even if branch exists.

Closes #2503

Raises PRs and includes warning if lock file updates fail and release timestamp is greater than a day old.

Closes #2233

Fixes #2201

This fixes a race condition where if someone merged multiple PRs in a row then a renovation-in-progress would get confused and post a “PR has been edited” message to an already-merged PR.

Closes #2115

This PR adds the feature of commenting on a failed automerge. It's done by adding a conditional in `lib/workers/branch/automerge.js` which, in case of receiving `failure` or `error` from the `getBranchStatus` function, returns the "branch status error" value. Another modification is in `lib/workers/branch/index.js`, which is an adition to the failure response of the `tryBranchAutomerge` function. The added functionality is the ability to add a comment to the PR which had a failure automerging. In case of receiving the aforementioned "branch status error" value, to the comment is appended a note which emphasize the fact that there're multiple failed status checks.

Closes #1934 

Corrected logic mistake and renamed unpublishable (internal) to canBeUnpublished to make it easier to follow logic.

Closes #2080

Renames currentVersion to currentValue, newVersion to newValue, newVersionMajor to newMajor, and newVersionMinor to newMinor.

Rewrite of dependency extraction, particularly for npm. Paves way for easier addition of new package managers.

Closes #1882