Enable aggressive-resize globally

Aggressive-resize was turned off because it isn't handled well by
iterm2's inbuilt tmux support.

As I'm using other terminals more often now, and I find iTerm's
bastardized support to be irritating (so much so that I've tweaked tmx
to never use it) I no longer care about being nice to iTerm.
aggressive-resize is such a useful feature that I can't use a terminal
that can't cope with it.