Make prompt configurable with env var

[gxmas]

Make the prompt configurable via the env var ZAP_PROMPT

For example,

ZAP_PROMPT='
%(?:%{$fg_bold[green]%}✔:%{$fg_bold[red]%}✗) % %{$fg[cyan]%}%3~%{$reset_color%}${vcs_info_msg_0_}
%{$fg[yellow]%}$%{$reset_color%} '

plug "zap-zsh/zap-prompt"