New @Config.Prefix annotation.

[Gmugra]

This is solution for #259

With new @Prefix annotation you can set prifix for all properties in the Config interface,
which helps to avoid @key annotations.

Was:

public interface ExpandsFromAnotherKey extends Config {

    @DefaultValue("dev")
    String env();

    @Key("servers.${env}.name")
    String name();

    @Key("servers.${env}.super.hostname")
    String hostname();

    @Key("servers.${env}.port")
    Integer port();

    @Key("servers.${env}.user")
    String user();

    @DisableFeature(VARIABLE_EXPANSION)
    @Key("servers.${env}.password")
    String password();
}

Now:

@Prefix("servers.${env}.")
public interface ExpandsFromAnotherKey extends Config {

    @DisableFeature(PREFIX)
    @DefaultValue("dev")
    String env();

    String name();

    @Key("super.hostname")
    String hostname();

    Integer port();

    String user();

    @DisableFeature(VARIABLE_EXPANSION)
    String password();
}

[coveralls]

Coverage increased (+0.1%) to 93.072% when pulling fb9db08 on Gmugra:issue_259_prefix into cc20db1 on lviggiano:master.