Add g:tagbar_jump_offset configuration (#695)

Closes #504

Add a new `g:tagbar_jump_offset` value which will control the tag jump location relative to the center of the screen.

plugin/tagbar.vim

@@ -96,6 +96,7 @@ function! s:setup_options() abort
         \ ['hide_nonpublic', 0],
         \ ['height', 10],
         \ ['indent', 2],
+        \ ['jump_offset', 0],
         \ ['left', 0],
         \ ['help_visibility', 0],
         \ ['position', default_pos],