Support Content Item Request and Response

Content item Request and Response are newer additions to the LTI specifications by IMS Global:

This allows content items to be selected and that selection information to be passed back to the tool consumer.

The specification defines 2 new lti_message_types

I am currently working on an implementation and should have a pull request soon.

I did want to mention how I was implementing the code so far.

There are several new params defined in launch_params
added an is_content_request method to tool_base
Made a new class called ToolOutbound that ToolConsumer inherits from as well as a new class called ContentItemResponse
ToolOutbound has the init, generate_launch_request, and generate_launch_data from ToolConsumer
- ToolConsumer keeps its has_required_params
- ContentItemResponse had its own has_required_params

pylti / lti