Add option for g:tagbar_highlight_method (#709)

* Add option for g:tagbar_highlight_current_tag

Closes #708

Add option for g:tagbar_highlight_current_tag which will allow
highlighting of tags on the current line in addition to highlighting the
scoped tags. Reverts the behavior added in #638 to the default from
before that commit

* Rework to allow direct configuration of highlight method

* Address review comments

plugin/tagbar.vim

@@ -100,6 +100,7 @@ function! s:setup_options() abort
         \ ['jump_lazy_scroll', 0],
         \ ['left', 0],
         \ ['help_visibility', 0],
+        \ ['highlight_method', 'nearest-stl'],
         \ ['position', default_pos],
         \ ['previewwin_pos', previewwin_pos],
         \ ['scopestrs', {}],