Add user_setting functionality

[chrishalcrow]

This PR allows the user to specify a settings.json file, to set custom settings for each view. The file looks like e.g.

settings.json

{
  "waveform": {
    "overlap": false, 
    "plot_selected_spike": false, 
    "auto_zoom_on_unit_selection": false
  },
  "probe": {
    "show_channel_id": false
  }
}

and you use it like so

sigui --settings-file settings.json my_sorting_analyzer

I don't really expect users to use this. Instead it's so that users can save their current settings, and reload them easily.