add ability to set custom prefix for cloud theme

themes/cloud.zsh-theme

-PROMPT='%{$fg_bold[cyan]%}☁ %{$fg_bold[green]%}%p %{$fg[green]%}%c %{$fg_bold[cyan]%}$(git_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
+if [[ -z $ZSH_THEME_CLOUD_PREFIX ]]; then
+    ZSH_THEME_CLOUD_PREFIX='☁'
+fi
+
+PROMPT='%{$fg_bold[cyan]%}$ZSH_THEME_CLOUD_PREFIX %{$fg_bold[green]%}%p %{$fg[green]%}%c %{$fg_bold[cyan]%}$(git_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
 
 ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[green]%}[%{$fg[cyan]%}"
 ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"