Toggle visibility of hidden files

This pull request adds a new command to toggle visibility of hidden files (#7). The visibility is changed basically by using glob qualifiers to include or not files starting with dot (`D` qualifier).

I needed to change also `deer-mark-file-list` function since it may happen now that passed arguments won't include marked directory when the parent directory is hidden and user presses key to hide dot files. If so then only first `$DEER_HEIGHT` of them are displayed. Also evil `eval` is used to avoid repeating perl program twice.