Add flags argument to vim/plugin/tagbar

Tagbar plugin supports 3 different formats for displaying a tag string.
A flags arguments was added to current_tag function to support this
feature.
About documentation: it is OK to reference tagbar documentation. But in this case it is better to add a reference to HTML variant of the documentation with proper anchor (if any) or, at least, github page that displays this documentation. In the latter case *do not add `#L{number}`* for referencing exact line numbers: these numbers will constantly get out of sync. Use something like “(search for ‘term’)” to reference it.