Generate extension accessors by default

[bamboo]

Before v0.10.1, extension accessors had to be explicitly enabled via a project property, this is no longer the case and accessors for any extensions introduced during the evaluation of the plugins block will be available by default. This means the following Gradle build script will now work as expected without any additional configuration:

plugins {
    application
}

application { // application extension accessor
    mainClassName = "my.App"
}

This behaviour can still be disabled via a Gradle project property:

org.gradle.kotlin.dsl.accessors=off

[eskatos]

LGTM 👍