Add support for custom format validators

[skonves]

When using the request validation middleware, support should be added for formats not defined at http://json-schema.org/latest/json-schema-validation.html#rfc.section.7. The proposed syntax would add a customFormats property to the middleware options:

app.use(gangplank.requests({
    swaggerDefinition: spec,
    customFormats: {
        uuid: value => validateUuid(value)
    }
}));

The validation error message could read similar to: "Validation failed per custom format 'uuid'" or further consideration could be given to allowing custom formatters to defined their own error messages.