`clean` task created only if it doesn't already exist. This will allo…

[noel-yap]

clean task created only if it doesn't already exist. This will allow the plugin to be usable along with other plugins (eg the Gradle java plugin) that create a clean task.

[palantirtech]

Thanks for your interest in palantir/gradle-npm-run-plugin, @november-yankee! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.