Access data fields on transformation functions

[zoedsoupe]

Description
Allows the access of other data fields while applying the transformation (aka mapper) function
on a :transform field, example:

import Peri

s = %{
  id: {:required, :string},
  name:
    {:string,
     {:transform,
      fn
        name, data -> (data[:id] && name <> "-#{data[:id]}") || name
      end}}
}

Peri.validate(s, %{id: 1, name: "maria"})
# {:ok, %{id, name: "maria-1"}}

It should also work on nested schemas (check test file) and handle errors.

Related Issues
closes #13

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Additional Context
N/A

[zoedsoupe]

improving some documentation stuff