Input / output setup

[smidf]

Partly depends on #22

the minimum Gradle plugin / task setting is e.g. the following:

configure<SwaggerCodeGenConfig> {
    sourcePath = "${project.projectDir.absolutePath}/openapi"
    outputPath = "${buildDir.absolutePath}/openapi"
    ...
}

val generatedPetStoreSrcDir = File(buildDir, "openapi/<name>/src/main/kotlin")
sourceSets {
    getByName("main").java.srcDirs(generatedPetStoreSrcDir)
}

1. I would set sourcePath to ./ instead of ./spec or ./specification
2. The vast majority of generators set the output automatically, e.g. <buildDir>/generated/source/<name>. This directory is also added to sourceSets automatically. I would assume that the generator will behave the same way.