Allow node and edge id's as integers.

[TimZaman]

Otherwise throws an error "Invalid format for 'id'. Must be a string,(...)". I guess integers are fine right.

Almost forgot to say, this repo was just what i was looking for, a minimal but highly effective elegant implementation, thanks.

[uruuru]

Good point, should definitely be allowed. Sadly it requires a change in the Java code, which I probably won't touch anymore until klayjs is migrated to ELK (the successor on the java side).

[uruuru]

Fixed in elkjs.