Add STAC output orders request

[kevinlacaille]

As the title reads, I've added STAC output to the Orders CLIs and APIs request function.

planet.cli.orders.py

• Added --stac as a click.option as a flag (i.e., boolean)
  • @cholmes, I've set the default to be false, but I could be convinced that it should be set to true

planet.orders_request.py

• Added stac as an input to build_request()

tests.integration.test_orders_cli.py

• Added an integration test for the Orders CLI for a STAC output

tests.unit.test_orders_request.py

• Added a unit test for the Orders API (build_request()) for a STAC output

Below is an example of the STAC metadata at work:

> planet orders request \                          
--name Bangladesh_2018_visual \
--id 20180418_043113_1053,20180418_043112_1053 \
--item-type PSScene \
--bundle visual \
--stac

{"name": "Bangladesh_2018_visual", "products": [{"item_ids": ["20180418_043113_1053", "20180418_043112_1053"], "item_type": "PSScene", "product_bundle": "visual"}], "metadata": {"stac": {}}}

FYI: @cholmes @sgillies @jreiberkyle @mkshah605

Closing #597

[kevinlacaille]

What's left?

• test STAC option in Orders CLI
• test STAC option in order_request.build_request()

[tschaub]

Looks good, @kevinlacaille. You can take or leave my comment as you like.