feat(version): add --force-git-tag option

[jzempel]

Description

Adds a new version --force-git-tag option which ensures any existing tag will be replaced rather than failing with an error.

Motivation and Context

Currently GitHub does not offer tag protection (isaacs/github#1091) meaning anyone with repo write access can add tags. We can purge these unwanted tags, but it is very easy for them to reappear as soon as someone with write access pushes stale tags back to the repo.

We're experiencing this issue with https://github.com/zendeskgarden/react-components. We want the option for lerna version to "win". The new option provides the means for asserting that the version tag generated by lerna version is, in truth, the desired tagged commit.

How Has This Been Tested?

• added associated unit test
• ./core/lerna/cli.js version -h execution lists the new argument

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[evocateur]

I'm leery of using any flag with "force" in its name, but I guess this doesn't hurt.