Allow hooks for listening and responding to un-resolvable skills

[manthanhd]

When a skill cannot be resolved, it throws an error. This shouldn't always be the case. It should allow a subscriber to intercept the unresolved skill call and act on it with a response. The listener should also be able to respond with a message and direct botjs to continue onto the next skill resolution.

[manthanhd]

Implementation might look like this:

var skill = new Skill(undefined, skillFn);

bot.addSkill(skill);

The resolver should map this to 'undefined' topic which will come up when classifier is trying to resolve topic. Also, for those whose skills cannot be found, the resolver will reduce down to this skill.

[manthanhd]

Closed by #9