Support for any/generic CI service

[Alorel]

I propose adding a "generic" CI service where greenkeeper-lockfile is configured entirely by environment variables:

• GK_LOCK_REPO_SLUG
• GK_LOCK_BRANCH_NAME
• GK_LOCK_FIRST_PUSH
• GK_LOCK_CORRECT_BUILD
• GK_LOCK_UPLOAD_BUILD

This would allow greenkeeper-lockfile to function in any environment, including Github actions. I would be happy to open a PR for this.

Considerations:

• The generic CI service would need to be used as a fallback and ignored if any other CI is detected.

What are your thoughts on this, Greenkeeper team?

[DanielRuf]

including Github actions

That would be great.