Try to support custom variable db options in admin

[bozdoz]

The idea is to have both key and value as textboxes (likely in a "custom variable" section). Where the user can add as many as they'd like.

---

| key | value |
| your-key | your-value |

and use it, for example:

[leafet-marker svg path="{ your-key | custom }"]

This could be parsed with the liquid function in https://github.com/bozdoz/wp-plugin-leaflet-map/blob/master/class.leaflet-map.php#L383

See how the map shortcode is currently implementing a 'raw' liquid-like tag: https://github.com/bozdoz/wp-plugin-leaflet-map/blob/master/shortcodes/class.map-shortcode.php#L193-L195