feat: add svgId option

[ouuan]

📑 Summary

Add a new option -I, --svgId [svgId] to set the id attribute of the <svg> element to be rendered.

This is useful when the output is used as a part of HTML, because Mermaid CSS pollutes the global CSS with the id.

📏 Design Decisions

svgId || 'my-svg' to use the old id as default. || instead of ?? to avoid empty id and for better compatibility.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted master branch

[aloisklink]

Looks great to me :)

Awesome that you also added a test case!

Edit: I'll leave this PR open for a few days (or until the next mermaid-cli release) in case anybody else wants to review it, before merging it.

[MindaugasLaganeckas]

@aloisklink you are welcome to merge it!