Add KISS way to run danger

# Report

## What did you do?

I want to run Danger. I don't really want all crazy env variables smartness and guessing which env variables do I need where.

My expectation is to be able to do something like this:

CHANGE_ID=123 GITHUB_TOKEN=123 REPO_URL=123 danger ...

Or (mind just 2 letters change)

CHANGE_ID=123 GITLAB_TOKEN=123 REPO_URL=123 danger ...

Instead, today I have to give `JENKINS_URL=qqq` for noone knows why reason and check with Danger source code for set of env variables it expects in each random case.

It's extremely frustrating to do not have simple way to run danger in the same way it runs in CI. I understand the motivation to make it easy to use from ci, but I just want to run it regardless, I prefer explicit over implicit and I will pass right variables, so I really would love to have simple way to do so. IF my PR is in Github, I want immutable command line which would work regardless of my underlying CI system, I prefer `danger --pr-id=123 --token=123 --repo-url=123` much more than `danger` doing many matches and semi-"intelligence" in the background, but still failing with "Not a Jenkins Pull request" or "Not a Github Actions pull request" (because I do have GITHUB_ACTION env variable set as webhook variable).

Would it be possible to have CI-independent way of running danger with fixed command line parameters?