feat: add length argument for password autogen

[ravihlb]

Adds -l | --length argument.

Exceprts from the updated README:

  -l LENGTH, --length LENGTH
                        Optional: Length of auto-generated password. Ignored if password argument already provided.

ex.

# -l is ignored since -p takes precedence
> ./pwpush -l 10 -p africanswallow
Pass is: africanswallow
URL: https://pwpush.com/p/zynqhemqkyvxi4q

> ./pwpush -l 10
Pass is: 0ezR.)cwg,
URL: https://pwpush.com/p/9ylhw57z4_qywugmqw