Add g:tagbar_scrolloff configuration (#692)

* Add g:tagbar_scroll_offset configuration
Closes #564

Add option for `g:tagbar_scrolloff` to issue a `setlocal scrolloff=#` during tagbar window init

* Address review comments

autoload/tagbar.vim

@@ -947,6 +947,10 @@ function! s:InitWindow(autoclose) abort
     setlocal nomodifiable
     setlocal textwidth=0
 
+    if g:tagbar_scrolloff > 0
+        execute 'setlocal scrolloff=' . g:tagbar_scrolloff
+    endif
+
     if g:tagbar_show_balloon == 1 && has('balloon_eval')
         setlocal balloonexpr=TagbarBalloonExpr()
         set ballooneval

doc/tagbar.txt

@@ -1025,6 +1025,21 @@ Example:
 >
        let g:tagbar_wrap = 1
 <
+                                                      *g:tagbar_scrolloff*
+g:tagbar_scrolloff~
+Default: 0
+
+If set to non-zero, the tagbar window initialization will set the |scrolloff|
+value local to the tagbar window to the specified value. This is used to
+position the current tag in the tagbar window. See the help for |scrolloff|
+for more details. If set to a very high value (greater than the height of the
+tagbar window), then the current tag should always stay in the center of the
+tagbar window.
+
+Example:
+>
+        let g:tagbar_scrolloff = 10
+<
 
 ------------------------------------------------------------------------------
 HIGHLIGHT COLOURS                                           *tagbar-highlight*

plugin/tagbar.vim

@@ -101,6 +101,7 @@ function! s:setup_options() abort
         \ ['position', default_pos],
         \ ['previewwin_pos', previewwin_pos],
         \ ['scopestrs', {}],
+        \ ['scrolloff', 0],
         \ ['show_balloon', 1],
         \ ['show_visibility', 1],
         \ ['show_linenumbers', 0],