Enable t.X functions

[mcnuttandrew]

There are a whole bunch of functions like .only and .skip which are disallowed by the trb implementation, it would be good if we could reproduce those. To be clear I'm referring to the functions that would in regular tape be invoked through commands like

tape.only('my cool test', t => etc)

A naive solution involves stapling additional functionality in through the test object syntax eg

{
     name: 'my cool test',
     test: t => t.ok(1, 'yep'},
     only: true,
     onFinish: () => console.log('cool dogs wear sunglasses')
}

Upside: this is really obvious and probably easy to use.
Downside: it probably means TRB will need to create it's own versions of each of those operators.