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.

Comienza rastreando cómo se validan los schematic schemas y cómo el prompt system gestiona la entrada de opciones; el issue nombra la schematic function como la solución alternativa actual. Se considera terminado cuando la validación personalizada definida por el schema puede cubrir casos entre opciones y basados en el sistema de archivos, validar la entrada durante el prompting y volver a solicitarla cuando la entrada no es válida.

Escrito por el modelo de indexación a partir del texto del issue.