Add vim-style <c-e> / <c-y> scrolling.
Add documentation for win-scroll-*
Simplify window_scroll_up.
Merge pull request #130 from wes1138/scroll

Add vim-style <c-e> / <c-y> scrolling.
cmus 2.6.0-rc0

Signed-off-by: Gregory Petrosyan <gregory.petrosyan@gmail.com>