implement planet orders request-update

Start with the request-update section of design-docs/CLI-Orders.md and the existing orders CLI entry points introduced by #616. Compare the command interface and options with the proposal, then verify that an existing order request can be updated and emitted in the requested JSON format, including pretty printing.

With #616, there is a new command, request-update

Implement with interface to match CLI-Orders proposal:

Interface
Usage: planet orders request-update [OPTIONS] REQUEST

REQUEST: order request to be updated. Can be string, filename, or '-' for stdin.

Update an order request.

This command creates a new order description from an existing order description, with certain entries changed according to the options specified. It outputs the updated order request, optionally pretty-printed.

Support for updating an order description is provided however it has many limitations compared to what the Orders API supports. For creation of more advanced order requests, create the order description manually or use the Python library to aid in creating the order description.

Note that if the order description contains a tool chain with more tools than just clip, using --clip will override the entire tool chain, not just the clip tool.

Options:

--clip JSON - Clip feature GeoJSON. Can be a json string, filename, or '-' for stdin. 
--tools JSON - Toolchain JSON. Can be a json string, filename, or '-' for stdin.
--email BOOLEAN - Send email notification when order is complete. --cloudconfig JSON - Credentials for cloud storage provider to enable cloud delivery of data. Can be a json string, filename, or '-' for stdin.
--pretty - Format JSON output.