Add `Leaflet.encoded` plugin: Enable creating PolyLine and Polygon from encoded string

[achieveordie]

Fixes #1886

• Create a plugin class called PolyLineFromEncoded, which accepts an encoded string to make a polyline. Also, add all the relevant docs/tests required for plugin approval.
• Create a plugin class called PolygonFromEncoded, which works the same way as above, but for polygons.

FYI @hansthen

[achieveordie]

In hindsight, I'm also considering adding the second functionality the original author provides - Polygon.fromEncoded(). They will be in the same plugin file (renamed from polyline_encoded to encoded) but have separate test/doc files.

[hansthen]

In hindsight, I'm also considering adding the second functionality the original author provides - Polygon.fromEncoded(). They will be in the same plugin file (renamed from polyline_encoded to encoded) but have separate test/doc files.

Do you want to include this in this PR? If so, go ahead, but you can also do it in a later PR.

[achieveordie]

done @hansthen :)

[achieveordie]

How long before this can be merged? I'd like to use the 'official' feature :)

[achieveordie]

@Conengmo I appreciate your input on the error. It was an oversight on my end. This PR should be good to go.