Support for extended macOS configurations

[mmenozzi]

As stated at https://github.com/amphp/artax/issues/150, it seems that macOS resolver looks at /etc/resolver directory to get the nameserver to use for a certain TLD. For example if you put the file /etc/resolver/dev with content 127.0.0.1, the local nameserver will be used for all the *.dev domains.

This behavior should be supported by this library.

[kelunik]

Just a note for future readers: This requires a configuration format change and is therefore a BC break. It might be considered to implement this in the same release as #70, but that one might not need a BC break depending on the solution.