Improve Testimony token validation

[elyezer]

Currently the validate command inspects if the minimum tokens are defined across all the test cases. Would be even better if Testimony allows users to define some validation rules like:

• Token's value is in a list of choices
• Token's value is unique (useful for validating a identification tokens)
• Token's value is matched by a regex
• Token's value is integer, float, etc...

To provide even more power each token can have one or more validation rules and Testimony must report all rules that have failed.

A proposed solution would be allowing the user to specify a path to a YAML file where the validation rules are defined for the validate command. The YAML file could be also a config file where the minimum tokens, tokens and other information can be defined.

[elyezer]

This was started on #142. More improvements will be done later.