Disable handlers before enabling

[lucaswoj]

… in order to prevent duplicate event listeners from being bound

fixes #2069

cc @mcwhittemore @bhousel @ansis

[bhousel]

Calling disable is a good idea, but I got the impression that #2069 was more asking for these methods to be written as accessor/mutators, so that other code could get the current handler state.

handler.enabled();
// returns true/false

handler.enabled(false);
// disables handler

[mcwhittemore]

@bhousel getters might be worthwhile, but the problem I was running into was too many callbacks being bound by calling enable when the handler was all ready enabled. This should solve that problem.

[lucaswoj]

Let's merge this as-is and create getters and setters if a use case emerges.

mo' state, mo' problems.