Set up a config file for the persistent options

[strboul]

• Create a config file (e.g. $HOME/.config/git-substatus/config.json) for some options.

If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

For example:

{
  "styles": {
    "repository": {
      "color": "blue",
      "emphasis": "bold"
    },
    "stash": {
      "color": "green"
    }
  },
  "components": {
     "item_bullet": "•",
     "display": {
       "directory": true,
       "branch": true,
       "stash": false
     }
  }
}