Make it possible to disable addResourceDir

[AB-xdev]

The problem

Currently THIRD-PARTY.txt always ends up inside the classpath root of my jar, but I don't need it there.

Cause

license-maven-plugin/src/main/java/org/codehaus/mojo/license/AddThirdPartyMojo.java

Line 187 in 42b3808

addResourceDir(outputDirectory, "**/*.txt");

Possible solution

Make it possible to disable this behavior using a flag (which is enabled by default for compatibility), e.g. named addOutputDirectoryAsResourceDir

[AB-xdev]

Did a quick PR that should resolve this: #580