Schematics schema should take custom validation functions

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Desired functionality
I would like to be able to specify validation functions for options of a schematic in the schema. JSON Schema does have some [built-in validation](http://json-schema.org/latest/json-schema-validation.html#rfc.section.6), but this doesn't cover everything.

Here are a couple of examples that are not covered by JSON Schema:

- An option for the name of a new directory or file that should not match the name of an existing directory or file
- An option whose validity is based on a previous option's value

A workaround is to perform this type of validation within the schematic function itself, however, this is not the best possible user experience when using the prompt system. As a user, I would expect my input to be validated as I enter it, then re-prompted for if necessary. With this workaround, a user would need to re-invoke the program from the beginning if their input is found invalid within the schematic function.

Commencez par retracer la manière dont les schematic schemas sont validés et la manière dont le prompt system gère la saisie des options ; l’issue désigne la schematic function comme solution de contournement actuelle. Le travail est terminé lorsque la validation personnalisée définie par le schema peut couvrir les cas entre options et ceux fondés sur le système de fichiers, valider les entrées pendant le prompting et redemander une saisie lorsque l’entrée n’est pas valide.

Rédigé par le modèle d'indexation à partir du texte de l'issue.