New setting 'cache_path' to override the default path for local caches and built tools

New setting 'cache_path' to override the default path used for the CabalInspector/ModuleInspector tools and local caches. The value can be a relative or absolute path (environment variables will be expanded appropriately).

This setting is useful for preventing auto-generated or inherently local files from being synchronized by Dropbox and the like.