Add some commands

I want to use 'ctrl+f' to go several next stories and 'ctrl+b' to go several prev stories.
But I found that it can't be accomplished,because urwid could not recognize them.(Ibash & mac book air)
So I mapped the two to 'f' and 'd'.
That is: use 'f' to go next  15 stories and 'd' to go prev 15 stories.
I alse mapped 'G' to go to the last story and 'g' to the first.
The detect_end function is based on the circumstances of showing only 60 top/newest stories and 30 best stories.