Add support for <l-tile-layer-wms> Custom Element.

• Use light DOM in accordance with existing Custom Elements.
• Support non-WMS query parameters by leveraging the underlying L.TileLayer.WMS options object
• Add unit tests to cover lifecycle events
• Add API docs to instruct users to use attributes correctly

Useful links

• Leaflet TileLayer.WMS API docs
• src/index.js to register new element
• src/l-tile-layer.test.js inspiration for tests
• src/l-tile-layer.js inspiration for new custom element