Allow to use a specific formatter while computing verification of a particular object #13

As per now, the framework relies on a toString call from the object under verification so it can be written in a file.

In some cases, that might not be convenient (if the toString method is already used for something else, or not providing the information you'd like). The framework should allow to specify a formatter function for that.

For example:

Approvals.verify(anObject, { o -> o.aField + " - " + o.anotherField });

Should allow to format the output not calling toString from the provided object, but calling the formatter function instead.

• format Map (order keys, one key/value per line)
• format collections (one entry per line)