[semver:minor] Optional checkout step

[adrogon]

• New step_checkout boolean parameter, default true: skips the checkout step is set to false
• New workspace_path string parameter, default "": adds an attach_workspace step if set

Closes #28

[Markl121]

Thank you @adrogon! Picked and merged with https://github.com/CircleCI-Public/continuation-orb/pull/36