Move example aliases to 'custom' section of the .zshrc template

By convention, user-specific aliases are kept in each user's `.zshrc` file. The .zshrc template provided by oh-my-zsh has an area for example aliases, though these were being loaded *before* other aliases in libs, plugins, and themes. As a result, personal aliases could be overwritten as other items are loaded by oh-my-zsh.

To make personal customization easier, the sample aliases section of the .zshrc template has been moved to the area dedicated for personal customization. This section of the configuration is processed after all other items are loaded, preventing personal aliases and exports from being inadvertently clobbered by oh-my-zsh.