Twig template name should follow naming conventions

Problem/Motivation

While working with Drupal 8 I experienced, that the twig file, does not follow the Twig naming conventions we have in D8. You can find a summary at Template naming conventions. We should change it before more and more people start to adopt D8 and the Leaflet module.

Proposed resolution

Use a dash instead of underscore for the file template and change the hook_theme() implementation.

leaflet_map.html.twig -> leaflet-map.html.twig