Make the rules optional

[aminnairi]

Is your feature request related to a problem? Please describe.
In case there is no rule to be applied to a schema, the library still forces the developer to write a rule.

Describe the solution you'd like
The rules property for each schema should be optional, and default to an empty array.

import * as Kryptonian from "kryptonian";

// This
Kryptonian.Kalel.string({
  message: ""
});

// Instead of this
Kryptonian.Kalel.string({
  rules: [].
  message: ""
});

Describe alternatives you've considered
None.

Additional context
This would require a tiny amount of handling in the source-code, but could potentially reduce the amount of code needed to be written in the developer side.