Create "IdentityCard" rule

[henriquemoody]

• v::identityCard(string $countryCode)

Validates Identity Card numbers according to the defined country.

v::identityCard('PL')->validate('AYW036733'); // true
v::identityCard('PL')->validate('APH505567'); // true
v::identityCard('PL')->validate('APH 505567'); // false
v::identityCard('PL')->validate('AYW036731'); // false

For now this rule only accepts Polish Identity Card numbers (Dowód Osobisty).

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Closes #623