Add a --files-from option

[hoelzro]

Normally, ack gets its list of files to search from ARGV. This feature would allows uses to specify a file containing one filename per line that specifies the list of files.

[hoelzro]

I wonder if there's a way we could also allow normal ack filters to be applied to this list? I found myself wanting the following today:

for file in $(ack --files-from=<(git ls-files --modified) --perl -f); do
  perl -c $file
done

(checks that all Perl files that have been changed compile)

[hoelzro]

This could also allow functionality like that requested in #35.

[hoelzro]

Expanding on how this would work with #35: we could have a way to alias option groupings, so ack --mtime=$mtime would translate to something like ack --files-from=<(find . -mtime $mtime), and ack --git-dirty would translates to something like ack --files-from=<$(git ls-files --modified).

[hoelzro]

This has been added.