pimp commentary
use cl-lib instead of cl

`cl' is deprecated in favor of `cl-lib'.

Since Magit requires `cl-lib' at run-time, we might as well
do the same here, instead of just requiring it a compile-time.
remove unnecessary autoloads

Since `magit' is required at run-time it is unnecessary to autoload
functions that it defines.
add to magit-extensions custom group

For now this group only exists on Magit's next branch,
but adding it now does not much harm.
fix doc-strings

Use imperative form and end with a period.
add magit-find-file-ignore-extensions

replacing `magit-find-file-skip-images' which used a constant list
of a small subset of image file extensions.  Even if that list were
"complete" this option would still be problematic because it is it
provides very arbitrary filtering; only a subset of potential users
of this package would want to filter images, while others might want
to ignore other kinds files.  For that reason use the empty list as
default value of the new option.
magit-find-file-completing-read: improve prompt and error message
magit-find-file-completing-read: refactor

* Absorb `magit-find-file-files'.
* Use functions from `dash',
* and drop dependency on `cl-lib.
* Don't use two local variables.