Added Decimal validations

[madclaws]

What?

Added validation functions for Decimals.

Why

Although Tarams library supports :decimal type, we were not able to do any validations against it like other types, since Valdi didn't have the validations defined.

How

After the added validations in Valdi, we can use the decimal validations in Tarams, for ex

    max_amount: [
      type: :decimal,
      required: true,
      decimal: [greater_than: Decimal.new("0.0"), less_than_or_equal_to: Decimal.new("99999")]
    ],

Testing

Added tests for all validation functions

[bluzky]

@madclaws Thanks for your PR, I've just release Valdi 0.4.0 and new Tarams version