Individual background color for components

[jumangee]

For now pintora component diagrams can be customized by theme only, but sometimes it can be needed to colorize components individually or by "stereotype". Plantuml example:

[hikerpig]

Thank you for the issue.
PlantUML's skinparam command and stereotype system is a little bit complicated and not quite the same across various type of diagrams.
Also, Mermaid.js has its own style keyword in some of the diagrams.

I'll need some time to think about how the custom styling shall be.