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

Adds capability to upgrade major versions of Go Modules. Includes special handling for gopkg.in modules.

Closes #2583

Closes #2589

Adds support to update dependencies that have pseudo-versions like v0.0.0-20140422174119-9fd32a8b3d3d.

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

Closes #2587

Adds support for updating go modules (found in go.mod) and the accompanying go.sum checksum files.

Limitations:

- github.com modules only (but supports "redirects" to github e.g. by gopkg.in and golang.org)
- skips major upgrades > v1
- no vendoring support

Closes #933, Closes #2578, Closes #2579