Fix ExternalLink class to support 'element-list' module definitions

[sjoerdtalsma]

This pull request intends to fix #142

There is also an issue in the maven-javadoc-plugin that does not resolve openjdk 11 documentation by default.
We can work around it by providing it explicitly, e.g.

<links>
    <link>https://docs.oracle.com/en/java/javase/11/docs/api</link>
</links>

That will include links in the documentation, but because the modules are part of the path (and no package-list is published, but an element-list instead) the UML classes do not contain links.

[sjoerdtalsma]

We'll need to figure out when to add the module name to the link path and when not to do that 😢

The JDK 10 documentation does contain an element-list (instead of package-list): https://docs.oracle.com/javase/10/docs/api/element-list

However, it does not include the module name in the documentation url:
https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html
(instead of https://docs.oracle.com/javase/10/docs/api/java.base/java/lang/Object.html)