Support for setFileInput() over Proxy

setFileInput() was implemented in #170. However, the setFileInput() does not currently work over the Proxy as expected because files are not transmitted from the local Chromeless instance to the Proxy (something which may be difficult given the AWS IoT 128KB message size limit.)

This issue tracks progress on supporting the Proxy.

Acceptance Criteria

• Add support to setFileInput() for S3 paths (which means fetching the S3 file, then passing the local disk path from the resulting object to setFileInput()
• Add support to setFileInput() for URLs
• if a local file path is provided to setFileInput() we will try to upload it to S3, then pass the S3 path to the Proxy.

await chromeless.selectFile('.uploader', [
  's3://my-bucket/my/object/prefix/file.jpg',
  'http://sweet-site.cool/my-image.jpg'
])