player: add scrolling

[jbohren]

It would be great if when the user used the scroll wheel on the web viewer, it scrolled up as if it were a real terminal window. This would be great for looking over output that might get printed really quickly.

(migrated from asciinema/asciinema#137)
@wjwwood

[SonGokussj4]

Hi. I've just found out about Asciinema and it's awesome. But right the first day this is a feature that's missing for me. Often there is much output and if viewer can't see the scrolling (either by mouse or by SHIFT+UP/DOWN or SHIFT+PGUP/PGDOWN, it's less usable.

[janluke]

First time user here. This was my first and only issue with asciicinema, but a severe one. As a workaround, one can pipe the program output to less or more but that's annoying and, at least on Windows (I'm using PowerSession), it removes colors.

[pronebird]

I am surprised this has not been implemented yet.

[ku1ik]

asciinema's virtual terminal emulator (avt) didn't support scrollback in the past, which was the blocker. Now the scrollback is there (since last year) it would be possible to have scrollback/scrolling implemented in the player. No promises though, maybe some day.