Add create, list-extensions and add-extension mojos

[cescoffier]

Example of usage:

# Create a new project from scratch - prompt the user
mvn org.jboss.shamrock:shamrock-maven-plugin:1.0.0.Alpha1-SNAPSHOT:create
# Note if there is already a project, it extends the `pom.xml` file

# List extensions
mvn shamrock:list-extensions

# Add extensions to the project
mvn shamrock:add-extension -Dextensions="Eclipse Vert.x, JPA"

This PR is related to #157

[stuartwdouglas]

I am ok with this as long as it is understood that this is just a short term thing for M1, that will be replaced later.

Thinking about it I am not actually sure the main repo is the place for this. Would there be any real downside to having these CLI style mojo's under a different plugin artifact?

[stuartwdouglas]

There should probably be managed in the parent pom

[cescoffier]

+1, will fix it.

[cescoffier]

If we move it outside of the main plugin, it needs another "prefix" and cannot reuse "shamrock" (or whatever it will be).

So this would not be possible:

mvn shamrock:list-extensions

It would be something like:

mvn shamrock-cli:list-extensions
mvn shamrock:dev

[stuartwdouglas]

From a usability perspective then I guess it is better to just have them all in the same module then.

[dmlloyd]

This isn't really our long-term solution though, right? Wouldn't we want to maintain a published extension list of some sort?

[cescoffier]

Definitely a placeholder for something better later. These commands are just there until we get a proper CLI.

For the extension, we will need a repository/registry somewhere. I've attempted to scan maven central, but it's not a good way (required > 24 hours to scan the artifacts...). I think this registry will be part of the web site.

[stuartwdouglas]

We definitely need a real discovery mechanism. I am happy to merge this as it makes it easy for people to get started when we do the internal reveal, however I this we need to make sure that everyone understands that this is a temporary solution.