custom extension options

Straight from the readme / changelog:

= 0.9.6 =

• Tweak: Added custom extension options via filter hook (ui_leaflet_add_extension_options), exposed in JS as global object ui_leaflet_extension_options

= 0.9.5 =

• Tweak: Added marker extraction routine for the main shortcode (in preparation for future versions)
• Tweak: Use filter hook to enforce assets loading (_ui_leaflet_load_assets)
• Added experimental custom icons (SVG / CSS)
• Several bugfixes