auto-revert-mode integration

Update diff-hl when auto-revert-mode-hook is called.
Merge pull request #30 from sunesimonsen/master

auto-revert-mode integration
Don't error out when fringe is disabled

Fixes #20 (belatedly) and purcell/emacs.d#241
diff-hl-dired-{unknown,ignored}: inherit from dired-ignored instead
diff-hl-dired-update-ignores: Specifically exclude .hg from iteration

'hg status' throws a fit when passed that name.
Add user option `diff-hl-dired-extra-indicators'
Use "i" for ignored files
diff-hl-dired: Always use dir-status-files

...halving the number of process calls.
Introduce diff-hl-dired-mode-unless-remote

Closes #31
Collect files only when diff-hl-dired-extra-indicators is non-nil
diff-hl-dired-update: Support Emacs 25

Fixes #28
Fixes #32