Add key binding to take command from region

[markbirbeck]

As described in #5 and #63 we need a key combination that executes the command under the cursor, or that is selected:

[
  {
    "keys": ["blah+de+blah"],
    "command": "shell_command",
    "args": {
      "command": "",
      "region": "arg"
    }
  }
]