Implement CI build command

We should implement some kind of CI build command that can be used to run plugin builds on a CI systems like Travis-CI and Jenkins.

The idea is that you can run something like `graylog-project ci-build` inside a plugin project and the tool creates an implicit manifest that includes the server and the current plugin project.

This would solve the problem we currently have that we manually have to clone the server repository in the CI "script" to be able to build web plugins.