Use oh-my-zsh's custom directory feature to enable my personal theme

Instead of using a specific fork with my theme added. For the time
being, I still need to use my own fork, as there was a couple of bugs
with customizing the custom directory path. I've created a fix and
waiting for the pull request to be merged:

https://github.com/robbyrussell/oh-my-zsh/pull/1111

shell/zsh/custom/plain.zsh-theme

@@ -0,0 +1,23 @@
+PROMPT='%n@%m %~$(check_git_prompt_info)$ %{$reset_color%}'
+
+local return_code="%(?..%{$fg[cyan]%}%? ↵%{$reset_color%})"
+RPROMPT="${return_code}"
+
+ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[cyan]%}("
+ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_DIRTY="%{$fg[red]%}*%{$fg[cyan]%})%{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_CLEAN="%{$fg[cyan]%})%{$fg[blue]%}"
+
+
+# Git sometimes goes into a detached head state. git_prompt_info doesn't
+# return anything in this case. So wrap it in another function and check
+# for an empty string. -- Brorrowed from Soliah.zsh-theme :)
+function check_git_prompt_info() {
+    if git rev-parse --git-dir > /dev/null 2>&1; then
+        if [[ -z $(git_prompt_info) ]]; then
+            echo " %{$fg[cyan]%}(detached%{$fg[magenta]%}*%{$fg[cyan]%})%{$reset_color%}"
+        else
+            echo " $(git_prompt_info)"
+        fi
+    fi
+}