add-repository should offer a built-in list of known repositories

We can anticipate most of the repositories the user is going to want to add (the JBoss Nexus repository being the most likely choice). add-repository should allows the user to select from a built-in list of well-known repositories if they don't provide a URL on the commandline. The list could even be read from the .forge directory, so that the user can globally define repositories from which they want to be able to select.

add-repository <ENTER>
Please select a repository:
[1] CENTRAL - http://repo1.maven.org
[2] JBOSS_NEXUS - http://repository.jboss.org/nexus/content/groups/public
[3] JBOSS_LEGACY - http://repository.jboss.org/maven2
[3] JAVA_NET - http://download.java.net/maven/2/
[4] CUSTOM

Selecting CUSTOM would prompt the user for a URL.

The repository name should have a reasonable default, which can be overridden by supplying a name on the commandline.