Result type common interface

[lbdm44]

As a consumer of ember-cli-checkup, when writing a new result type, I should have access to a common interface that defines the different methods of output (i.e. CLI vs JSON) so I can create custom result types when writing new tasks/plugins.

[lbdm44]

@suchitadoshi1987 I think you should take this one on instead as a follow up to #122 JSON output