ATHENS (Reuters) - Greece wants to continue negotiations for a cash-for-reforms deal with its creditors based on the proposals it has submitted, which the government believes are the best basis for talks, a Greek government official said on Saturday. Greek Prime Minister Alexis Tsipras has called a referendum on austerity demands from foreign creditors, rejecting an "ultimatum" from lenders and putting a deal that could determine Greece's future in Europe to a popular vote. ...