Allows installing to subdirectories of GameData

Fixes #315 .

If the install stanza contains a path like:

    "install_to": "GameData/path/to/subdirectory"

then the target file is installed to the appropriate directory.
Total nitpick, absolutely not a PR-blocker: one could probably use `if (install.install_to != "GameData")` without needing to check its length here. :)