Add feature which allows users to make the plugin insert a new line after the header license.

Hello!

I've just started working with this absolutely awesome plugin, however, even after going through the source codes of it, I was not able to figure out one thing. And maybe I am just extremely dumb, or I might have also overlooked something (okay, this is no different from being dumb), but really, how do people do this? I mean, is there any way to make the plugin insert a new line just after the end of the license header? Or, am I perhaps the only one who finds this feature crucial?

Seriously, I consider this:
```java
/**...*/
package nl.javadude.gradle.plugins.license;

public class Test {

    public static void main(String[] args) { ... }

}
```

extremely annoying. This:
```java
/**...*/

package nl.javadude.gradle.plugins.license;

public class Test {

    public static void main(String[] args) { ... }

}
```
is much better in my oppinion.

Anyway, I decided to implement this feature myself and just wait for your thoughts on this matter.

The changes I made were all tested in many different cases and should work just fine.

Cheers!