Adds configurable key bindings using rc.

See [documentation](https://github.com/dominictarr/rc)

The appname is 'vimdebug' and key bindings are under vim.keys.<command>, e.g.:
`vim.keys.break`.

For example, to configure the add-breakpoint key to <Ctrl-b>

`./lib/vim-inspector.js --vim.keys.break="C-b"`

or a `~/.vimdebugrc` containing:

```json
{
  "vim": {
    "keys": {
      "break": "C-b"
    }
  }
  // vim: set ft=json:
}
```

Closes #13.