Provide a support for Africell telecom operator in Angola

[Sharykhin]

Looks like phone numbers from a new telecom operator Africell doesn't work.

Phone number example: +244951949757

Go test:

package libphonenumber

import (
	"testing"
)

func TestPhoneNumber(t *testing.T) {
	phoneNumber := "+244951949757"
	p, _ := Parse(phoneNumber, "")
	if !IsValidNumber(p) {
		t.Error("phone number is invalid")
	}
}

Output:

=== RUN   TestPhoneNumber
    phone_number_test.go:11: phone number is invalid
--- FAIL: TestPhoneNumber (0.00s)
FAIL
FAIL    github.com/heetch/libphonenumber        0.313s
FAIL