add finally call

Adds the `finally` call (loop independent).

Can be used for cleanups after test where `after` is unsuitable.

```lua
it('provide dummy stuff and clean up',function()
  local f = io.popen('stupid_process')
  -- ensure that once test has finished f:close() is called
  -- independent of test outcome
  finally(function() f:close() end)
  ... -- do some stuff involving f
end)
```

The same behavior could be accomplished with describe / before / after, but this would bloat code alot and more "distant" upvalues (`f`) would be required:

```lua
describe('when using dummy stuff',function()
  local f
  before(function()
    f = io.popen('stupid_process')
  end)
  
  after(function()
    f:close()
  end)

  it('do stuff with f',function()
    ...
  end)
end)
```