Autocompletion and navigation for attributes

• autocompletion for attributes and attribute references (:attr: and {attr}) (thanks to @bbrenne) (#277)
• renaming and find-usage for attribute names (#243)
• upgrade to AsciidoctorJ 2.1.0 and Asciidoctor 2.0.10
• statement completion adds newline if at end of file (#276)
• listing and other delimiters recognized at end of file (#276)