Add struct-level custom rule

[lasantosr]

This PR would close #61

It's backwards compatible so a minor release would be enough

[lasantosr]

@jprochazk This is implemented following your guidelines from #61 (comment), can you have a look and generate a new version?

The main use case are mutually exclusive options like you mentioned, but there are other custom validations that could be performed.

Manually implementing the Validate trait would require a lot of boilerplate for structs that contains lots of fields with lots of validations the derive macro is already capable of managing, because you would have to manually call all of those validations and manage errors and paths.