Adds --ask option to confirm before exiting msfconsole

The goal is to help people when they forget where they are and accidently type "exit" on msfconsole.
New msfconsole -h entry:
 *    -a, --ask                        Ask before exiting Metasploit or accept 'exit -y'

New prompt when typing 'exit' or 'quit' at msfconsole with --ask specified.  It accepts variations of y and yes, defaults to no, and is bypassed with 'exit -y':
 *   Are you sure you want to exit Metasploit? [y/N]: 

Validated by testing "exit" and "exit -y" with and without the --ask option and with/without active sessions.