feat: trigger external repo

[KyleTryon]

Aims to resolve #14 by enabling the action to trigger additional CircleCI projects beyond the current repository.

This is useful, for instance, if you have a remote repository set to build on the latest version of a docker image, you can use this action in the repo which builds the docker image, to test it with the remote repository immediately.

New optional inputs

  target-slug:
    required: false
    description: 'The slug of the target CircleCI project. For example, "github/<org>/<project>". 
Will default to the current project. Can be overwritten with "TARGET_SLUG" environment variable.'
  target-branch:
    required: false
    description: 'The branch of the target CircleCI project. Will default to the current branch name. 
This should be overwritten if "target-slug" is set. Can be overwritten with "TARGET_BRANCH" environment variable.'
  target-tag:
    required: false
    description: 'The tag of the target CircleCI project. Will default to the current tag name if set. 
This or branch should be overwritten if "target-slug" is set. Can be overwritten with "TARGET_TAG" environment variable.'

[brivu]

LGTM!