Opt-In to Leaflet-Geoman

Minor Changes

• Added opt-in Functionality: #504
  You can now initiate leaflet-geoman with an opt-in functionality which basically sets pmIgnore: true as the default to all layers. See the docs for more info.

Patches

• Upgraded to Webpack 4 & fix security issue: #500
• Handles snapping for added shapes during draw. Fixes #477: a685d96
• Finishing a Rectangle now respects snapped marker positions. Fixes #448: 7caad97
• Style changes during edit mode are now properly preserved . Fixes #470: #506
• Added migration instructions: c00b70f