Extract browser-related abilities to dedicated package #344

It seems that open has a lot of code, dependencies, and issues and feature requests dedicated to browsers, even though those are just a fraction of the available applications.

Would it make sense to publish a dedicated package to deal with custom URL needs? It would use open as a sub-dependency to actually run the command I suppose.

This would match the existence of a dedicated package to open editors, for example: https://github.com/sindresorhus/open-editor (which depends on open and does not need all the logic related to browsers)