Allow checking of SHASUMs when downloading files

[MrHedmad]

For large files, integrity is not always guaranteed. Allowing Kerblam! to handle checking some kind of hash sums after running kerblam data fetch would be ideal.

This would mean having new options in the kerblam.toml file. I'd suggest something like this:

[data.remote]
"url_to_fetch" = "target.txt" 

[data.remote.hash]
"hash" = "target.txt"

or, alternatively

[data.remote]
"url_to_fetch" = {
    to = "target.txt",
    hash = "hash"
}

The second option would either need some custom parsing logic or to release kerblam 2.0. But It would be nice to not have to set the hash every time.

Adding a new command like kerblam data hash that generates the code to insert in the kerblam.toml to check the hashes based on the currently downloaded files would also be nice.