Delay task loading until it's necessary

This change fixes #25 by delaying task loading until the task is actually run, or the CLI is asked to list the available tasks. Tasks are still located as part of dullard's startup process, but aren't actually required until they're run.

When the tasks are loaded so the CLI can list them, we ignore any errors that may be caused by side effects from loading a module, since we're not really interested at that point.

All of the tests pass, and dullard seems to be running fine for me after this change.