[Feature] Include custom types in schema validation

Like so:

import { addCustomTypes } from 'typy';

addCustomTypes({
  isFirstName: input => 3 <= input.length && input.length <= 15,
  isLastName: input => 3 <= input.length && input.length <= 15
});

import t, { Schema } from 'typy';

const schema = { 
  firstName: Schema.isFirstName,
  lastName: Schema.isLastName
}

t({
  firstName: 'John',
  lastName: 'Doe'
}, schema);