Added multi-threaded capability & file parameters

The script is now multi-threaded, so the user can determine how many
threads to occupy during script execution. The provides a significant
speed increase, which on my end increased from 1 account/second to 18
accounts/second at a measly 12 threads!
The user can also specify the file to load the usernames from. A final
note, the algorithm for finding accounts was changed from a separate
username and password list, to using the password as the username, and
incrementing an integer onto the username. This yields many more
results.