Add support for customFields

This version adds support for customFields in the config.
Thanks to @aadgrant for the contribution and @coreyward for the help.

Example with custom fields added:

{
  "theme": "dark",
  "themeChanges": {},
  "customFields": [
    {
      "label": "Additional description",
      "name": "description",
      "placeholder": "No description yet...",
      "type": "textarea"
    },
    {
      "label": "Decade when photo captured",
      "max": 2200,
      "min": 1800,
      "name": "decade",
      "placeholder": "Between 1800 and 2200",
      "step": 10,
      "type": "number"
    },
    {
      "label": "Is a premium photo",
      "name": "premiumPhoto",
      "type": "checkbox"
    },
    {
      "label": "Attribution",
      "name": "attribution",
      "placeholder": "No attribution yet"
    },
    {
      "name": "location",
      "label": "Location",
      "type": "location"
    }
  ],
}