Implement IsURL util func, remove govalidator dependency

[cpu]

The github.com/asaskevich/govalidator dependency could be removed from both zcrypto and zlint if zcrypto provided its own IsURL utility function.

Alternatively, removing the three invocations of govalidator.IsURL in x509/json.go would accomplish similar.

zcrypto/x509/json.go

Lines 458 to 462 in 0afe7b2

	for _, name := range c.URIs {
	if govalidator.IsURL(name) {
	jc.Names = append(jc.Names, name)
	}
	}

zcrypto/x509/json.go

Lines 464 to 469 in 0afe7b2

	for _, name := range c.IPAddresses {
	str := name.String()
	if govalidator.IsURL(str) {
	jc.Names = append(jc.Names, str)
	}
	}

zcrypto/x509/json.go

Lines 557 to 563 in 0afe7b2

	// Check for wildcards and redacts, ignore malformed urls
	if strings.HasPrefix(name, "?.") || strings.HasPrefix(name, "*.") {
	ret = isValidName(name[2:])
	} else {
	ret = govalidator.IsURL(name)
	}
	return

[cpu]

Relates to #183 ?