Use tab width of 2 spaces for JS.
Add descriptions to Zsh startup files (from http://zsh.sourceforge.net/Intro/intro_3.html)
Source additional Zsh startup files from `$ZDOTDIR/*.d` directories. Rename `rc.d` to `zshrc.d` for consistency.
Move exports from `zshrc.d` directory to `zshenv.d`.
Source Prezto configuration from `zprezto.d` directory.