Default charset for PlantUML

[cbmek]

Hi, is it possible to introduce charset configuration, required by PlantUML, to this plugin?

Right now platform's default charset is being used:
https://github.com/NetworkedAssets/git4c/blob/master/confluence-plugin/src/main/java/com/networkedassets/git4c/infrastructure/plugin/converter/plantuml/PUMLConverterPlugin.kt#L24

Sometimes your repo/files may have different charset then platform running confluence. In that case, PlantUML diagrams are rendered without some UTF-8 (i.e. latin extended) characters.