Docopt usage. Settable savepaths. requirements.txt

Lookie here. Fucked your code up again ;_; Sorry.

* `docopt` for help text and argument parsing
  * `-d|--directory` to set savepath (`~/8chan`) at runtime
  * `-s|--subject` to set subject (`~/8chan/SUBJECT`) at runtime instead of asking
* added `requirements.txt` with some explanations in `README.md`