Add explain command

from https://github.com/schneems/explain_shell/blob/master/bin/explain.sh
Add shortcut for Twitter app positioning
Replace .ackrc with .agignore
Merge branch 'master' of github.com:justincampbell/.dotfiles
Add Messages and Twitter positions
Retry command
Merge branch 'master' of github.com:justincampbell/.dotfiles
Merge branch 'master' into simpler-prompt