Add support for a NEVER_DESCEND option

Ok, as I asked in #8, I've come up with the solution (actually, two approaches):

+ `show_dir`
+ `show_file`

I personally liked the second because it's looks better in the output and the code makes more sense (at least to me :laughing:).

```shell
# ~/rcrc
EXCLUDES="README.md systemd zsh.d"
NEVER_DESCEND="zprezto dircolors"

~/d/s/rcm git:master ❯❯❯ lsrc
/home/pablo/.Xmodmap:/home/pablo/.dotfiles/Xmodmap
/home/pablo/.autostart:/home/pablo/.dotfiles/autostart
/home/pablo/.dircolors:/home/pablo/.dotfiles/dircolors (not descending)
/home/pablo/.gemrc:/home/pablo/.dotfiles/gemrc
/home/pablo/.gitconfig:/home/pablo/.dotfiles/gitconfig
/home/pablo/.gitignore:/home/pablo/.dotfiles/gitignore
/home/pablo/.rcrc:/home/pablo/.dotfiles/rcrc
/home/pablo/.ttytterrc:/home/pablo/.dotfiles/ttytterrc
/home/pablo/.xinitrc:/home/pablo/.dotfiles/xinitrc
/home/pablo/.zlogin:/home/pablo/.dotfiles/zlogin
/home/pablo/.zlogout:/home/pablo/.dotfiles/zlogout
/home/pablo/.zprezto:/home/pablo/.dotfiles/zprezto (not descending)
/home/pablo/.zpreztorc:/home/pablo/.dotfiles/zpreztorc
/home/pablo/.zprofile:/home/pablo/.dotfiles/zprofile
/home/pablo/.zshenv:/home/pablo/.dotfiles/zshenv
/home/pablo/.zshrc:/home/pablo/.dotfiles/zshrc
~/d/s/rcm git:master ❯❯❯
```

Of course, the commits are sequential so if this is going to be merged the unwanted approach has to be removed. If you like it, I can clean it up so you can squash everything in one commit later.

PS.- It's a pain working with the `man/*` files, is there a better way to work with them instead of trying to guess what the hell it's going to happen at the end?