Config dirs for vim and zsh

Allow for additional configuration in subdirectories: `~/.vim/configs` and `~/.zsh/configs`.

Under `.vim/configs` you might find: some Swift-specific settings that you and the iOS team are trying; email-specific settings that you and other mutters are sharing; general one-off settings that you want scoped and well-named.

Under `.zsh/configs` you might find: the latest Ruby switcher that you're trying, some cool keybindings that you and your friends can't live without, or Android-specific settings that you're sharing with the other Arch users.

`.zsh/configs` supports a `pre` and `post` subdirectory, so you can have more control over the order in which things load. For example, you may want to set `chpwd` in a `post` subdirectory to prevent odd `cd` behavior while your rc files load.