Set initial terminal window size

[WPprodigy]

Refreshed version of https://github.com/derailed/k9s/pull/1280 (rebased, fixed merge conflicts, addressed feedback). Fixes https://github.com/derailed/k9s/issues/1267

The exec ends up looking something like this:

/usr/local/bin/kubectl --context mycontext exec -it -n mynamespace pod-name -c php-fpm -- env COLUMNS=173 LINES=29 sh -c command -v bash >/dev/null && exec bash || exec sh

Replicating the terminal sizing issue is a bit strange though. I consistently had the terminal sizing issues forever, but upon first trying out this patch it became a bit inconsistent even when not using the patched version. I'm going to use this branch as my default, and will report back if it ever crops back up again.

[derailed]

@WPprodigy Thank you Caleb for this pr! Looks like we have conflicts...