Feature Request: Ignore anonymous option (#727)

Closes #726

New option `g:tagbar_ignore_anonymous` to ignore any tags with __anon in
the tag name. CTAGS will print any unlabeled tags with the __anon###
name with a custom hash. This option will ignore those tags and they
will not be visible in the tagbar window. This could potentially result
in unknown behavior when a parent object has been ignored in this way.
However these tags should then fall under the default hierarchy for that
particular kind.

plugin/tagbar.vim

@@ -101,6 +101,7 @@ function! s:setup_options() abort
         \ ['left', 0],
         \ ['help_visibility', 0],
         \ ['highlight_method', 'nearest-stl'],
+        \ ['ignore_anonymous', 0],
         \ ['position', default_pos],
         \ ['previewwin_pos', previewwin_pos],
         \ ['scopestrs', {}],