Add gateway verification of Dat links

[chartgerink]

Dat links originally have three components, namely the protocol (dat://), the 64 character hash, and a version number (+3 for example, optional). However, with services like hashbase.io (maybe also datBase in the future), this heuristic is insufficient to verify Dat links and needs to be extended.

There are two options available:

1. Create a list of services that would have to be curated manually (unknowing how many services like #_hashbase pop up in the next months/years)
2. Resolve the links via the Dat protocol (but how?)

Any comments appreciated.

Ps on 2. Dat-cli seems to do this already so there might be some hints in there (or as always, we could use a system call but I want to prevent unnecessary system calls if we know things would break beforehand)

[chartgerink]

It seems like Dat uses Dat-node as its fundamentals, so it could be worthwhile to check that out and work with that at some point for improved efficiency and resolve this verification.