Allow custom generators (#847)

* Extract generatedJsonAdapterName to public API for other generators/consumers

* Fix kapt location in tests

* Add IDE-generated dependency-reduced-pom.xml to gitignore

This always bites me

* Add generator property to JsonClass and skip in processor

* Opportunistically fix formatting for generateAdapter doc

* Extract NullSafeJsonAdapter for delegate testing

* Add custom adapter tests

* Allow no-moshi constructors for generated adapters

* Fix rebase issue

* Use something other than nullSafe() for lenient check

This no longer propagates lenient

* Add missing copyrights

* Add top-level class note

* Add note about working against Moshi's generated signature

* Add missing bit to "requirements for"

* Note kotlin requirement relaxed in custom generators

* Style

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessor.kt

@@ -85,7 +85,7 @@ class JsonClassCodegenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils
   override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
     for (type in roundEnv.getElementsAnnotatedWith(annotation)) {
       val jsonClass = type.getAnnotation(annotation)
-      if (jsonClass.generateAdapter) {
+      if (jsonClass.generateAdapter && jsonClass.generator.isEmpty()) {
         val generator = adapterGenerator(type) ?: continue
         generator.generateFile(generatedType)
             .writeTo(filer)