conditionals shouldnt use eval by default

[cressie176]

{
  condition: '@some.property',
  task: 'foo'
}

throws an exception because the condition is evaluated rather than looked up from the interpolated. This is counter intuitive as most other task attributes are interpolated.

Howabout...

1. when condition is a function, execute it, passing the context as the first argument. Only support synchronous functions
2. when the condition is a string starting with @ interpolate it
3. when the condition is an object support an eval option, e.g.

{
  condition: {
    eval: 'Date.now() > 1435048419884'
  },
  task: 'foo'
}

[PeterAronZentai]

What should it do with...

1. condition is an object but not eval
   -> if(object) would always be true...
2. none of the above
   -> simple if(condition)