Make the url parameter non-required

Currently the url parameter to the AnnouncementMojo is required. This is not suitable for all projects.

As far as I can tell the url parameter is only used for two things:

1. Act as a stand-in value for the introduction parameter, if that is not supplied
2. Forwarded to the Velocity template for use there

Our use case is that we have a project that doesn't have a url. We still want to email an announcement when a new version is released. We are forced to enter a dummy url in our plugin configuration to get around this.

I'd like to remove the @required annotation from AnnouncementMojo.url. Are there any objections to this?

Projects already using the plugin will not be affected, because they already have a url specified in their configuration.