Add --file flag to request command to load JSON payload from a local file rather than provide it as an argument

[robbieaverill]

Resolves https://github.com/chec/cli/issues/20

Helpful when using the request command to upload an asset, for example, where the file contents need to be a base64 encoded string. This may easily be too long for bash, so you can use --file=/path/to/payload.json instead.