Tabline: Highlight unselected modified buffer

Use the modified airline_c color on unselected modified buffers in order
to highlight them as modified while unselected.
Tabline protect against missing palletes

If normal_modified or normal_modified.airline_c aren't present, fallback
to the default normal.airline_c.
Merge pull request #595 from mseabold/upstream

Tabline: Highlight unselected modified buffer
Add buffer_idx_mode to vim-airline.
Update documentation.
Add buffer_idx_mode to vim-airline.