Allow 6-digit postal code for Cambodia

[omega3000]

Cambodia (Code: KH) introduced a new 6-digit postal code format in 2018. While the old 5-digit format is still widely used and also still accepted by DHL International (Paket), validation should pass both formats. DHL Express for instance accepts the 6-digit format only.

Please refer to: www.grcdi.nl/gsb/cambodia.html khpostcode.com

I will do a pull request that extends the current rule and add it to unit tests accordingly.

The same issue occurs to Serbian postal codes, see Respect#1304