Rework FPATH and completion system

Rework FPATH and completion system

Now AX-ZSH manages its own additional FPATH components, and adds these to
the actual FPATH after reading all zprofile files and before handling all
the zshrc files.

This allows plugins to add custom FPATH directories in their zprofile
part, and to configure completion settings in their zshrc file: "compinit"
is called by the ax.zsh script between these two steps.