Add diff-hl-amend-mode

Closes #11
Add the footer
diff-hl-amend-set{,up}
diff-hl-amend: Add explicit (require)
Initial implementation of diff-hl-dired-mode

#6
Autoload diff-hl-mode
diff-hl-amend-mode: Reword the docstring
* Use one diff-hl-dired-process-buffer per dired buffer.
* Use `vc-responsible-backend', remove `diff-hl-dired-files'.
* Highlight subdirectories depending on files they contain.

Closes #6
Highlight unregistered files in diff-hl-dired
Autoload diff-hl-dired-mode
diff-hl-dired-highlight-items: Use the `single' type

* Because having changes in several disjoint files is the most usual
  situation, and they look better this way.

* It doesn't look too good with several adjacent changed files. Maybe
  do some smart grouping preprocessing later.
diff-hl-amend-mode: Don't defer via find-file-hook

Simpler model at the cost of +1 external process call (#11).