[Fix #477] Add instructions for updating Prezto
[Fix #479] Start only one ssh-agent instance
Add `custom` folder plus scripts and aliases.
Activate custom scripts and customise `.zshrc`
Add `prompt_jakob_setup` file and activate prompt.
Customise `zshenv`.
Customise `zpreztorc`.
Update external modules
Update alias definitions.
Update alias definitions.
Enhance aliases.
Enhance zshrc to be used on other boxes as well.
Enhance zshr (add if checks for named dirs etc)
Enhance aliases
Small change to prompt_jakob_setup.