Control output better

[chbrown]

Fix regression(-ish) in caeba7b that allows files to write wherever they want.

Ideas

• Default to refusing to overwrite existing files
  • require a -f / --force flag to clobber files
  • still might make a mess, despite averting egregious destruction
• Add -n / --dry-run option to preview what files would be written
  • would allow user to finagle destination (cd or via --output flag) to accommodate the results
  • still requires foresight to use it in the first place
• Relativize / rename files targeting paths outside destination
  • e.g., replace ../src/myfile.js with something like ./PARENT-src/myfile.js