help:describe should accept "cmd" argument

In a lot of cases, people want help about Maven commands without really knowing exactly what their plugin's artifactId/groupId is. Many people at my work are perennially unclear about the distinction between a lifecycle phase like "compile" and a plugin goal like "compiler:compile".

help:describe should allow the user to specify an arbitrary command with "cmd", they could then type "mvn help:describe -Dcmd=install" to get information about the install lifecycle phase, or "mvn help:describe -Dcmd=compiler:compile" to get information about the compiler plugin's compile goal.