Add template parameters to JsCode objects

Is your feature request related to a problem? Please describe.
I'd like to be able to refer to other folium /leaflet objects inside a JsCode object. So far I have been using string interpolation inside the body of the JsCode object.

Describe the solution you'd like
I'd like to add parameters to the JsCode __init__ method, together with a basic string templating solution. For the string templating I'd prefer to use the good-old %(name)s format.

Describe alternatives you've considered
I considered using the newer formatting templates such as used in python format, or Jinja templates. However, due to brace heavy nature of javascript, I think a completely new variable marker will lead to more readable code.

Implementation
folium is maintained by volunteers. Can you help make a PR if we want to implement this feature?

まず、JsCode init エントリポイントと、JsCode 本体で現在使われている文字列補間を読みます。要求されている %(name)s パラメータの動作を、既存の JavaScript 生成アプローチと比較します。JsCode がパラメータを受け取り、提案された基本的なテンプレート形式を使ってそれらをレンダリングし、新しい動作のカバレッジが追加されていれば完了とします。