stack-to-nix: provide functionality and ease-of-use of stack2nix

[rvl]

stack2nix is quite easy to use.

• PR stack-to-nix: Allow custom output filenames nix-tools#29
• Add an option for single-file output (rather than a directory of files), just like stack2nix.
• No need to implement stack2nix with URL to a stack file. I don't think anyone wants that.

[rvl]

Implemented in part by input-output-hk/nix-tools#29.

[stale]

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