feature: pull templates in from a URL

[andrewrynhard]

In microservices architecture you can have multiple applications written in the same language. For users/companies that want to standardize the CI/CD pipeline, they need a way to write tasks, stages, pipelines, scripts, etc. in one place to keep things DRY. We should support sourcing any of these from a URL. For example, we could read a task from a gist.