Make the height of stderr panel configurable

[quark-zju]

From a user (internal link: https://fburl.com/uqroyr2t):

My terminal is normally about 15 lines tall and eating almost half of this for noncritical information is pretty frustrating.

Perhaps make it configurable and have a default based on terminal height.

[markbt]

You can press Esc to hide it for now. I'll look into adding config.

[quark-zju]

Some users (ref https://fburl.com/ub3gar6l) want zero of the stderr or progress bar stuff. Perhaps also make them configurable.

[quark-zju]

I sent some diffs internally to merge stdout and stderr into a single pane (done outside sp). That might address most concerns here without extra configuration.

Might be good to optionally embed the progress output into the status bar (with a way to expand it) to keep the interface simple.