Add ability to set custom tags

[HelLViS69]

Hello,

it seems this plugin is not configurable at all. I've added the new JDK tags (apiNote, implSpec and implNote) and in subprojects they are generated correctly with the following code:

javadoc {
	configure(options) {
		tags(
			'todo:X',
			'apiNote:a:API Note:',
			'implSpec:a:Implementation Requirements:',
			'implNote:a:Implementation Note:'
		)
	}
}

This is a snippet of javadoc.options of one of the subprojects:

-tag 'todo:X'
-tag 'apiNote:a:API Note:'
-tag 'implSpec:a:Implementation Requirements:'
-tag 'implNote:a:Implementation Note:'

The problem is that when aggregateJavadocs is run, there is no way to add this configuration, the javadoc.options file misses those flags, and the task fails with error: unknown tag: apiNote