Support for Cane metrics

I've added support for [Cane](https://github.com/square/cane) to MetricFu

Cane can be configured using the following options in MetricFu:

```
MetricFu::Configuration.run do |config|
  config.cane = {:abc_max => 15, :line_length => 80, :no_doc => 'y'}
end
```