detect if git.io permalink is already taken
tmux: clear screen with `C-a C-l`
remove outdated ghi script
`git unreleased` shows commits made since last tag
`tmux-session save/restore` to help tmux through reboots

Currently only saves tmux sessions, windows and their current
directories. Ideally it would save exact pane layouts and reopen
programs such as Vim preloaded with last edited file.
benchmarking scripts: `lineprof` and `stdev`