add simple command history

[UltCombo]

• [x] Up/down arrows to move between commands
• [x] Do not push the command to the command history if it is identical to the previous one
• [x] Limit command history (e.g. max 500 commands, delete older first)
• [x] Add persistency to command history (so it survives through Atom restarts)
• [x] Figure out what is the best way to integrate the history with multiple sessions (is the command history shared by all sessions or each session gets its own? After closing all sessions and starting a new one, what command history will it use?)

[UltCombo]

On iTerm2, each session has its own command history, and creating a new session gets a merged command history ordered by the commands' chronological order from all previous/current sessions.