Added keybinding to export org to mail from buffer to subtree

~org-mime-htmlize-subtree~ as has the ability to set the email =To=, =From=,
=Subject=, =CC=, and =BCC= fields based on properties. If you commonly capture
and email the subtree being able to bake these settings into your template is
very convenient.

https://github.com/org-mime/org-mime#m-x-org-mime-org-subtree-htmlize

~org-mime-org-subtree-htmlize~ is similar to ~org-mime-org-buffer-htmlize~ but
works on subtree. It can also read subtree properties =MAIL_SUBJECT=, =MAIL_TO=,
=MAIL_CC=, and =MAIL_BCC=. Here is the sample of subtree:

  ,,* mail one
   :PROPERTIES:
   :MAIL_SUBJECT: mail title
   :MAIL_TO: person1@gmail.com
   :MAIL_CC: person2@gmail.com
   :MAIL_BCC: person3@gmail.com
   :END:
  content ...

CHANGELOG.develop

@@ -2456,6 +2456,7 @@ Other:
     - Added ~SPC m s a~ to toggle archive tag for subtree
     - Added ~SPC m s A~ to archive subtree (previously ~SPC m s a~)
   - Added ~SPC m i L~ as =org-cliplink= into =org= layer (thanks to bb2020)
+  - Added ~SPC m e s~ =org-mime-org-subtree-htmlize= (thanks to Nick Anderson)
   - Documented more insertion bindings (thanks to Lorenzo Manacorda):
     - ~C-RET~   Insert heading at end of current subtree
     - ~C-S-RET~ Insert TODO heading at end of current subtree

layers/+emacs/org/packages.el

@@ -652,7 +652,8 @@ Headline^^            Visit entry^^               Filter^^                    Da
       (spacemacs/set-leader-keys-for-major-mode 'message-mode
         "em" 'org-mime-htmlize)
       (spacemacs/set-leader-keys-for-major-mode 'org-mode
-        "em" 'org-mime-org-buffer-htmlize))))
+        "em" 'org-mime-org-buffer-htmlize
+        "es" 'org-mime-org-subtree-htmlize))))
 
 (defun org/init-org-pomodoro ()
   (use-package org-pomodoro