Use static method for value reading instead of extension method

This is a rather big change. It changes how the configuration value for a feature switch is being accessed from a custom switch implementation. Instead of typing `this.ReadSwitchFromConfig` and accessing the value via an _extension method_, now the user is advised to access the value using `FeatureSwitch.FromConfig` static method.

The reason for this is a great comment from @amacal, who pointed out that an _extension method_ actually should do anything (reasonable) with the type it is extending. Since the current implementation of the extension method does not do anything with the extended type, it is sort of counter-intuitive to use the extension way at all. Instead, one might expect a separate value access implementation.

I'm not sure about this. Here we claim **the** name predestinated for an actual **implementation** of a feature switch (which we actually do not provide). However, this static class does something very generic when parsing a boolean app config setting. I'm not even sure if we should provide this trivial functionality at all in the library. It would make sense for me though if we uncluded an actual abstract `AppConfigFeatureSwitch` base class, so creating concrete feature switches would be as easy as inheriting from it and providing the app settings key to the base class constructor. But however, I also agree with @amacal on his objection.