Default `TriggerRuleEvaluation` configuration to True

[atheiman]

Defaulting this configuration to True will generally make testing easier. Ideally this could default to True if a rule is only evaluated periodically, but this is difficult to evaluate from the DescribeConfigRules api (mix of attributes have to be checked differently for managed and custom rules. Not worth the complication.

There is a very small cost associated with invoking StartConfigRulesEvaluation (~ $0.001 per rule evaluation), so if this API is invoked by default the cost implication should be noted on the README.

Dont implement until after #7