set up highlighting when hovering over tree nodes
set up highlighting when hovering over tree nodes

fixes #77

citation-style-editor/csl-editor,adam3smith/csl-editor,fbennett/csl-editor,citation-style-editor/csl-editor,adam3smith/csl-editor,citation-style-editor/csl-editor,adam3smith/csl-editor,citation-style-language/csl-editor,fbennett/csl-editor,citation-style-language/csl-editor,citation-style-language/csl-editor