NewMultiFileReader constructs a MultiFileReader.

Parse parses the data in a http.Request and returns a command Request object.

TODO: make configurable.

The Handler struct is funny because we want to wrap our internal handler with CORS while keeping our fields.

MultiFileReader reads from a `commands.File` (which can be a directory of files or a regular file) as HTTP multipart encoded data.

Client is the commands HTTP client interface.