Allow specifying a custom views path

Currently, the views path is hardcoded (https://github.com/LinkedDataFragments/Server.js/blob/v2.2.2/lib/views/HtmlView.js#L34). If we make this an option, people can specify their own views.

This would be useful for custom views such as DBpedia's (http://fragments.dbpedia.org/, which I currently implement as a fork), and for other cases such as #61.