Autocomplete ecs task definitions

[sublimino]

Would be great to have these auto-completed:

ecs describe-task-definition --task-definition <task>

can be populated with

aws ecs list-task-definition-families and/or aws ecs list-task-definitions

[donnemartin]

@sublimino Thanks for the suggestion to add this. The resource completions will undergo some changes soon (https://github.com/donnemartin/saws/issues/36), once that's wrapped up we'll be able to more efficiently add autocompletions.