In addition to addFeature(), we need isFeature(id) and removeFeature(id)

[BurkovBA]

Hi, thanks for this library (it's very nice)! We're using it in RNAcentral.org now and are pretty happy about it.

I have a feature request: sometimes, we want to add a feature only if it's not already present. Or sometimes it could be nice to remove and existing feature.

As you're using a factory function to construct your FeatureBrowser and make its features attribute private, it's impossible to access features directly without corresponding API methods.

Cheers!