[feature] Implement simple saving SVG schemes

[iviaks]

This PR tries to close #62

I've used the same implementation as @mjbcopland provided in https://github.com/APIs-guru/graphql-voyager/issues/62#issuecomment-41251608 and located button inside PanelHeader

It's a simple implementation, because not included styles inside SVG file

---

Source:

Result:

[FluorescentHallucinogen]

@iviaks Is it possible to preserve CSS styles in the resulting SVG?

[iviaks]

Hey, @FluorescentHallucinogen According to article, we can add styles into svg.

It works for me in a next way.

1. source image;
2. screenshot from native Linux viewer;
3. screenshot from Chrome

If it's OK, we can leave it.

[tspike]

This has been open for 3+ years, any hope of getting it merged?

[aexol]

GraphQL Editor can export schemas