Rewrite using Vintage's hotkey architecture.

- Features: now handles space and escape to cancel out of Jumpy
- Features: now performs jump on second character (do not need to hit enter
  after entering keys).
- Readability: pulls out the duplicate_contents steps into a function to
  make more readable.
- Cleanup: Removes unused get_current_pos method
- Cleanup: Adds some TODOs
- Technical: Uses Vintage's architecture which has a static set of
  keybindings that are only applied with the application of certain
  "modes".  This is a built in feature in Sublime.
- I originally tried by utilizing the on_change and on_cancel of
  show_input_panel.  Ran into issues of closing early on 2nd key press.
  Also had issues with the cancel manipulation as well.  This keeps it
  closer to vim like behavior and Vintage's architecture.