Add tab_modified_indicator to the vim segments

This indicates in the tabline that any of the open buffers in a tab
page has unsaved modifications (like the default tabline do).
@janLo I was thinking that this key is always present, but it appeared that it and some other keys are not. Was fixed in #1007 and also made powerline not call contents_func when segment will be disabled due to its absence in `include_modes`.