[FEATURE request] hope `org-agenda-custom-commands` can support propertiezed-text.

[stardiviner <numbchild@gmail.com>]

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I want to display text icon using "all-the-icons" package to beautify
org-mode commands interface.

In `org-capture-templates`, I can set description text with propertied text
like this:

```elisp
(setq org-capture-templates
      `(("c" ,(format "%s\tsticky note" (all-the-icons-faicon
"sticky-note-o" :face 'all-the-icons-green :v-adjust 0.01))
         entry (file "")
         ;; select todo keyword interactively from `org-todo-keywords'.
         ;; The `org-todo-keywords-for-agenda' variable is fullfilled with
value AFTER generated Agenda.
         "* %(completing-read \"Todo keyword: \"
org-todo-keywords-for-agenda nil t) %^{Capture} [/] \n:PROPERTIES:\n:TIME:
%U\n:END: \n%i\n%a\n\n%?\n"
         ;; :time-prompt t
         :empty-lines 1 :jump-to-captured t))
```

But I can't do similar in `org-agenda-custom-commands` description part
like this:
```elisp
(add-to-list
   'org-agenda-custom-commands
   '("c" (format "%s Today [c]locked tasks." (all-the-icons-faicon
"hourglass-start"))
     ((agenda ""
              ((org-agenda-ndays 1)
               (org-agenda-span-1)
               (org-agenda-use-time-grid t)
               (org-agenda-include-diary nil)
               (org-agenda-show-log (quote clockcheck))
               (org-agenda-clockreport t))))))
```
This upper code will report error.

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