Don't wipe out b:airline_head if fugitive finds the head

- Fugitive can edit virtual files that represent objects in git; while
  editing these files, it still reports on the HEAD of the relevant
  repo, which should be displayed as the branch (rather than having the
  branch section disappear)
Merge pull request #577 from qstrahl/master

Don't wipe out b:airline_head if fugitive finds the head
Add `readonly` indicator also for !&modifiable

While typically both are set (e.g. with `:help`), I am using the
modifiable setting alone to prevent editing of some files via an
autocommand (`*.css`, if `*.scss` exists).

I can also set `readonly` for this buffer, but that gets reset when
reloading the buffer, which is not the case for `modifiable`.
Merge pull request #586 from blueyed/readonly-for-nomodifiable

Add `readonly` indicator also for &nomodifiable
Tabline close button flag added
Tabline: Highlight unselected modified buffer

Use the modified airline_c color on unselected modified buffers in order
to highlight them as modified while unselected.
Merge pull request #591 from igorpetrovcom/tabline-close-button-flag

Show/hide setting for close button in tabline
Tabline protect against missing palletes

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

Tabline: Highlight unselected modified buffer
Add buffer_idx_mode to vim-airline.
Update documentation.
Keep buffer index-id map a script scope variable.
Merge pull request #589 from paulhybryant/master

Minor enhancement for tabline
Merge pull request #598 from wesQ3/pr

Added extension for wesQ3/vim-windowswap
improve buffer index mode and fix usages where there's overflow
Handle special characters in part conditions
Merge pull request #603 from kballard/part_condition_special_char

Handle special characters in part conditions