Add start | end date for FEATURE flags

[compojoom]

Sometimes we need to enable a feature flag at a particular time. Other times we need to disable a feature flag at a particular time. I think that it would be beneficial if feature flags had a date field for start and end values. By default they should be null, which means the feature is null. If a date is entered then we should check the current date against the start and end time to determine if the feature is on or off.

Right now someone with access to the config service needs to be available to press a button at a specific time...