Create ParseFileWithPlantUmlTask.groovy

In one of my project, I needed the ability to parse a few files individually via plant uml. Here is a task that you could add to your plugin to make it easer.

I use it this way : 

```gradle
task createOverviewWithPlantUmlTask(type: ParseFileWithPlantUmlTask) {
  sources = [project.file('overview.html')]
  destDir = project.file("$buildDir/javadoc/")
}
```