Add a function to download an artifact that was previously uploaded

Current situation

A workflow can upload artifacts using the actions/upload-artifact function. But there is no way to get such artifacts from other jobs in the workflow.

Desired outcome

A download-artifact function that could be use to download an artifact in the execution environment.

Something that could be used as follows:

steps:
- uses: actions/download-artifact@v1
  with:
    name: my-artifact         # at most one of 'name', 'pattern'
    pattern: a pattern
    path: in-this-directory   # optional, workspace if unspecified

The step fails in error if the artifact is not found

name is optional, if not specified, all uploaded artifacts are downloaded.