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

baseBranch will be applied in PR title and in branch name only if there are more than one.

Closes #2306

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

Closes #1882