Add hidden setting (AlternateMouseScroll) for arrow keys on mouse scroll

The hidden defaults setting AlternateMouseScroll turns on "mouse wheel scrolling support when a terminal is in MOUSE_REPORTING_NONE mouse mode and is using an alternate screen by translating them to arrow up/down terminfo escape sequences. (ala MouseTerm)"

Activate it with
```
$ defaults write com.googlecode.iterm2 AlternateMouseScroll -bool true
```

This patch was submitted and adapted in https://code.google.com/p/iterm2/issues/detail?id=974 and then wrapped in a hidden setting by me