Password length argument

This pull request adds a **new argument** for the **password length** to generate (see #25).

-  `-s <LENGTH>` /  `--length <LENGTH>` (`-l` and `-n` are already used)
- A minimum length of *8* is required to prevent short and unsecure passwords *(this is open for discussion)*
- If there's no length given, the default value of `64` is used (this maintains the current behaviour)