Update to use escomplex-js for analysis

The `complexity-report` project is no longer recommended for direct use and has been refactored into components. This updates Plato to use the [escomplex-js](https://github.com/philbooth/escomplex) directly.

Please see [this issue](philbooth/complexity-report#32) for more information about the change.

This also moves some fixture data out of a string an into a fixture file, it was easier to make this change while having it to visually inspect.