Move aliases to 'custom' section of .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 by these
other aliases as they are loaded. 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.