Added support for other schemes

[Stilch]

For example

class WsUri extends Uri
{
    protected const SUPPORTED_SCHEME = [
        'ws' => 80,
        'wss' => 443,
    ];
}

[coveralls]

Coverage remained the same at 100.0% when pulling 37da6c8 on Stilch:feature-other-schemes into 78a2f6e on slimphp:master.

[andrew-demb]

Should it be static:: here?

Suggested change

	if (!key_exists($scheme, self::SUPPORTED_SCHEMES)) {
	if (!key_exists($scheme, static::SUPPORTED_SCHEMES)) {

[Stilch]

Yes, you're right. I fix it.