Introduce a mechanism to enable and disable plugins explicitly

[obfischer]

Userstory

As a user of jQA,
I would like to have a mechanism to enable and disable plugins of jQA explicitly,
so that I have a better control of the plugins jQA runs during a invocation.

Rationale

jQA offers a wide range of plugins for scanning, running concepts and constraints and for various reports.
Currently we run all of them during a single invocation of jQA. Each plugin we run prolongs the execution time and the might increase the size of the used Neo4j database, even if the result of a plugin is not needed by the user.
Allowing the user to enable of disable plugins gives him more control over these aspects and can result in a faster scan or a smaller database. The advantage of a smaller database is also that it might become easier to query the database as it might contain fewer labels or nodes.

Acceptance Criteria

• Each core plugin has a unique id and for the user easy to remember identifier
• Each plugin can by explicitly disabled
• Each plugin can by explicitly activated
• Having an id is optional for a plugin to avoid problems with third party plugins should issue a warning
• If jQA finds an plugin without an id jQA
• Without any given configuration jQA will run all plugins
• It is possible to disable all plugins and to enabler some plugins afterwards explicitly
• The meachnism is well documented