Editing shortcuts and keybindings

[felipeochoa]

This PR adds a few keybindings that automate some common JSON edition operations:

• C-c C-t: Toggle between true and false at point

  1. before: "options": {"alwaysInclude": |true, ...}
     after: "options": {"alwaysInclude": |false, ...}
  2. before: "options": {"alwaysInclude": |false, ...}
     after: "options": {"alwaysInclude": |true, ...}

• C-c C-k: Replace the sexp at point with null

  1. before: "options": {|"alwaysInclude": true, ...}
     after: "options": null|
  2. before: "question": {"number": |5, "type": "number"},
     after: "question": {"number": null|, "type": "number"},
  3. before: "question": {"number": 5, "type": "nu|mber"},
     after: "question": {"number": null, "type": null|},

• C-c C-i: Increment the number at point
  before: "question": {"number": |5, "type": "number"},
  after: "question": {"number": |6, "type": "number"},

• C-c C-d: Decrement the number at point
  before: "question": {"number": |5, "type": "number"},
  after: "question": {"number": |4, "type": "number"},

[joshwnj]

@felipeochoa thanks!