--output-dir option doesn't work

[iantruslove]

If you specify the --output-dir option, the JSON analyses overwrite the original APKs.

$ docker run -it --rm -v /output:/tmp/output:rw /apks:/apks:ro rednaga:apkid -o /tmp/output -v /apks/foo.apk
Traceback (most recent call last):
  File "/usr/local/bin/apkid", line 11, in <module>
    load_entry_point('apkid', 'console_scripts', 'apkid')()
  File "/apkid/apkid/main.py", line 94, in main
    scanner.scan(input)
  File "/apkid/apkid/apkid.py", line 111, in scan
    self.options.output.write(results)
  File "/apkid/apkid/output.py", line 101, in write
    with open(out_path, 'w') as f:
OSError: [Errno 30] Read-only file system: '/apks/foo.apk'

It's due to using an absolute path as the last arg to os.path.join -- e.g.

>>> os.path.join("/a/b", "/c/d")
'/c/d'