Diffoscope: in-depth comparison of files, archives, and directories

If someone is listening, I would suggest adding a SYNOPSIS section to the documentation with the command-line usage. e.g., does it support 3-way base/local/remote diffs?

`git difftool` knows about a lot of these kinda tools.. if this one looks promising (it's free, so that's already a plus) then we can teach difftool about it. I guess I'll wait and see if it materializes.

Adding support to git is pretty simple: create `mergetools/diffoscope` with this (I'm guessing the cli syntax, but you get the idea):

    diff_cmd () {
        "$merge_tool_path" "$LOCAL" "$REMOTE"
    }

    merge_cmd () {
        if $base_present
        then
            "$merge_tool_path" --output "$MERGED" "$BASE" "$LOCAL" "$REMOTE"
        else
            "$merge_tool_path" --output "$MERGED" "$LOCAL" "$REMOTE"
        fi
    }

Open API for differs (totally incomplete, but we've already released some its pre-reqs): https://github.com/diffplug/opendiff

Archives and filesystems that we support: https://docs.diffplug.com/1.6.1/Filesystems/