Add Json support for keybindings, to add own keybindings

[x7airworker]

I would like to write my own keybindings for iota, for example in Json.
Example:

[
  {
    "keys": [
      {
        "mode": "Ctrl",
        "key": "x"
      },
      {
        "mode": "Ctrl",
        "key": "c"
      }
    ],
    "CommandInfo": {
      "command_name": "editor::quit",
      "args": null
    }
  }
]

[gchp]

Sounds great! This is something I've been working towards. I outlined it at a high-level in this commit message: 7cf0e4f

Definitely something I'd like to have too.