cytoscape should be a peer dependency

[cappslock]

Rather than bundling cytoscape, it would be nice to express it as a peer dependency pegged to the major version. I'm happy to provide a PR for this.

[maxkfranz]

A peer dependency would not be appropriate within the semantics of package.json. This is a proper dependency, not a plugin dependency.

[akx]

Seconding this.

The same way react and react-dom are peer dependencies here, cytoscape should be.

I bumped into a problem where .use()ing a Cytoscape plugin didn't work because there were multiple versions of Cytoscape bundled in, and useing the plugin only registered it in the Cytoscape not being used by react-cytoscapejs.