what

Add the ability to map multiple ports.

why

Mapping multiple ports was not possible today.

Other things to note:

This is a breaking change, as it does away with the individual "container_port", "host_port", and "protocol" configuration in favor of providing the entire "port_mappings" list of maps.
This PR changes how environment variables get processed. They are now processed separately and replaced as the last step of output processing. This was done because the current solution was turning the ports into strings, which is invalid configuration for ECS. This effectively rolls back #9 and implements that solution in a new way that is compatible with the port mappings.