Add ALIAS support for INWX provider

[ngrie]

Is your feature request related to a problem? Please describe. While migrating my DNS zones to INWX, I noticed that there is no ALIAS support for that provider in DNSControl:

printIR.go:113: ERROR: domain xxxx.com uses ALIAS records, but DNS provider type INWX does not support them

INWX however does support ALIAS records using the web interface and the API.

Additional context From briefly scanning the code, I would guess that updating this line: https://github.com/StackExchange/dnscontrol/blob/8f18f6e7a1942fcf25bb2169eba05ea6df0a29f6/providers/inwx/inwxProvider.go#L46

as well as this part: https://github.com/StackExchange/dnscontrol/blob/8f18f6e7a1942fcf25bb2169eba05ea6df0a29f6/providers/inwx/inwxProvider.go#L172-L180

could be enough to add support? (I am not used to do Go programming so ...)

[cafferata]

Ping @patschi, the maintainer of this provider.