Add programmatic navigation and error message

[mitchdennett]

Adds the ability to for a component to programmatically call next.

this.$emit('can-continue', {value: true});
this.$emit('next', {value: true});

Also adds the ability for a component to listen to an error message. In before-next-step you can call next('Some error message', false); and then a component can watch error and then display the error message as needed.