Check whether markdown-command is installed

[syohex]

Markdown is not rendered if markdown-command is not installed. This causes unexpected test failure. Tests which are introduced at #102 are failed if markdown-command(Default value is markdown) is not installed. Body should not be evaluated if the command is not installed.

For example, test is failed if markdown is not installed because buffer content is not HTML and is bash: markdown is not found instead.

Test test-markdown-ext/live-preview-follow-min-max condition:
    (ert-test-failed
     ((should
       (=
        (window-point)
        (+ final-pt ...)))
      :form
      (= 36 44)
      :value nil))

CC: @cosmicexplorer