Add symbol highlighting for Neovim LSP

[lwyw]

Suggest adding LspReference... for symbol highlighting in colors/onedark.vim

  " Neovim LSP (for versions < 0.5.1) {{{
  hi link LspDiagnosticsDefaultError DiagnosticError
  hi link LspDiagnosticsDefaultWarning DiagnosticWarn
  hi link LspDiagnosticsDefaultInformation DiagnosticInfo
  hi link LspDiagnosticsDefaultHint DiagnosticHint
  hi link LspDiagnosticsUnderlineError DiagnosticUnderlineError
  hi link LspDiagnosticsUnderlineWarning DiagnosticUnderlineWarn
  hi link LspDiagnosticsUnderlineInformation DiagnosticUnderlineInfo
  hi link LspDiagnosticsUnderlineHint DiagnosticUnderlineHint
  call s:h("LspReferenceText", { "bg": s:visual_grey })
  call s:h("LspReferenceRead", { "bg": s:visual_grey })
  call s:h("LspReferenceWrite", { "bg": s:visual_grey })
  " }}}