Improvements to Command Box

[kuychaco]

• support additional commands and flags
• add --help support
• dropdown menu for loading pre-set scenarios
• make it resizeable
• add refspec support (head~, head^) Better refspec support #15
  • Prior ref versions and checkout history (@{n}, @{-n}, ref@{n} — requires reflog support)
  • refs/heads/<branch> (see Can't checkout branch name if a commit starts with the branch name #19)