ox-md: Fix image transcoding

* lisp/ox-md.el (org-md-link): Do not confuse caption and alt-text.
  Provide "img" as default alt-text.
Merge branch 'maint'
org-feed.el: Add config variant for current buffer

* lisp/org-feed.el (org-feed-alist): When FILE is nil the target
becomes the current buffer each time the feed update is invoked.
org-src.el (org-edit-src-code): Throw a warning instead of an error

* org-src.el (org-edit-src-code): Throw a warning instead of
an error when loading the mode fails, otherwise the user is
left with unusable buffers.

Thanks to Florian Beck for suggesting this.
Merge branch 'maint'
add :cmdline support to shell code blocks

* lisp/ob-shell.el (org-babel-execute:shell): Pass the cmdline header
  argument to `org-babel-sh-evaluate'.
  (org-babel-sh-evaluate): Pass the cmdline header argument to
  `call-process-shell-command'.