Add ability to have checkstyle configurations specified at the folder level

[ItaloBorrelli]

Although all the properties are allowed with "scope": "resource" a checkstyle configuration can't be provided at the folder level it seems. I would like to be able to have multiple checkstyles specified for a multi root osgi project with overrides available. I've provided an example repository here: https://github.com/ItaloBorrelli/checkstyle-example. Please comment for any clarification needed.

[ItaloBorrelli]

Having looked around at the repo the pattern I had in mind (and could implement myself for this project) is to

• Have the DelegateCommandHandler manage multiple instances of the checker service dependent on the path. If there is no configuration file is removed it will default to a parent.
• Handle checkstyleConfigurationManager.onDidChangeConfiguration a little differently by looking at the scope during the ConfigurationChangeEvent.affectsConfiguration and deciding how to handle refresh on that basis.

Let me know if you have any thoughts on this, or if you anticipate any issues with this.

[jdneo]

Looks good. One thing might be help. All the workspace root path should be available via JavaLanguageServerPlugin.getPreferencesManager().getPreferences().getRootPaths()