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.

Inizia tracciando come vengono validati gli schematic schemas e come il prompt system gestisce l’input delle opzioni; l’issue indica la schematic function come workaround attuale. Il lavoro è completato quando la validazione personalizzata definita dallo schema può coprire i casi tra opzioni e quelli basati sul filesystem, validare l’input durante il prompting e richiedere nuovamente l’input quando non è valido.

Scritto dal modello di indicizzazione a partire dal testo della issue.