Better buildSrc support

[ychescale9]

Fixes #5.

• Disabled IR to support applying the plugin within buildSrc using kotlin-dsl.
• Exposed AppVersioningPlugin as public API to support applying plugin using type instead of plugin id.

with(project) {
    pluginManager.apply(AppVersioningPlugin::class.java)
    plugins.withType<AppVersioningPlugin> {
        extensions.configure<AppVersioningExtension> {
            overrideVersionCode { _, _ ->
                Instant.now().epochSecond.toInt()
            }
            overrideVersionName { gitTag, _ ->
                "${gitTag.rawTagName} (${gitTag.commitHash})"
            }
        }
    }
}