Formatting; no changes to logic.
Use the `path` module to build file paths.

Use the `path` module to build file paths rather than ad-hoc string
concatenation. All this currently does is use the proper path separator on
non-UNIX platforms, but the structure is nicer and you can wrap the `path`
module to do things like prevent directory traversal vulnerabilities if this
script ever is being run in a security-sensitive context (read: by a
non-developer).