Generate printable cold storage of bitcoins

Changes for 1.12

• new option in paper-wallet: -w, --words - choose the number of words
• new option in paper-wallet: -c, --compat_addrs - choose the number of compat addresses
• new option in paper-wallet: -s, --segwit_addrs - choose the number of segwit addresses
• changed the way the main generator method handles optional arguments
• added a warning if the entered entropy is very short