Add --output-filename to output generated objects to

[terraboops]

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Fixes #3

• file written to tmp location initially and moved to the final path
• currently writing all objects to the same file, but this could be changed
  • writing different objects to different files will require passing a bunch of flags or using predefined filenames with a user defined prefix, this seems inconvenient for maintenance as well as for users

[terraboops]

Please let me know if there's anything else I can improve or fix with this PR! Thanks @liggitt :)