switch to my own extension tag in README.md and options.html

sed -i 's@eejcbegfnjfjnmdikkplhbhnemddchbn@cfeknjlpekkmneffdgocfncbjnbfhkmm@' README.md
sed -i 's@eejcbegfnjfjnmdikkplhbhnemddchbn@cfeknjlpekkmneffdgocfncbjnbfhkmm@' options.html