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:

dnscontrol/providers/inwx/inwxProvider.go

Line 46 in 8f18f6e

providers.CanUseAlias: providers.Cannot("INWX does not support the ALIAS or ANAME record type."),

as well as this part:

dnscontrol/providers/inwx/inwxProvider.go

Lines 172 to 180 in 8f18f6e

	/*
	INWX is a little bit special for CNAME,NS,MX and SRV records:
	The API will not accept any target with a final dot but will
	instead always add this final dot internally.
	Records with empty targets (i.e. records with target ".")
	are not allowed.
	*/
	case "CNAME", "NS":
	req.Content = content[:len(content)-1]

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

[cafferata]

Ping @patschi, the maintainer of this provider.