Flag to log only the failing conditions

[SamyPesse]

When using targaryen as the jasmine integration, when a test is failing, the output is really verbose for complex rules.

To debug the issue, we have to look for => false in all the logs.

I tried playing with setVerbose and setDebug, but none seems to help (either we have no logs or too much logs).

I'm suggesting adding an option to log only the failing conditions (the ones returning false or throwing an error).

[dinoboff]

I will try to tackle it when I have time but I am happy to accept a PR. TODO:

• [ ] it will need a new flag.
• [ ] a filter property on the Result class
• [ ] and filter logs with it