[PATCH] ox-md.el: enhance example/src blocks support

[Lungang Fang <lungang.fang@gmail.com>]

* lisp/ox-md.el (org-md-example-block):
Use "```" instead of 4-whitespace indentation for example/src blocks.
The 4-whitespace has difficulty handling blocks under list items.
The new code also allows syntax highlights for Programming languages

The following org mode snippet is a test input:

- Python src block within a list item
  #+BEGIN_SRC python
    for each in collection.find():
        count += 1
    print "Total number:", count
  #+END_SRC

#+BEGIN_EXAMPLE
  OOPS! This is a first level example block
#+END_EXAMPLE

Output of the old version of org-md-example-block does not show as expected
in Salsforce case editor or MacDown (http://macdown.uranusjr.com).

In comparsion, it is coverted to the following by the new version and is
handled by Salsforce and MacDown as expected:

-   Python src block within a list item

    ```python
    for each in collection.find():
        count += 1
    print "Total number:", count
    ```

```
OOPS! This is a first level example block
```

TINYCHANGE
---
 lisp/ox-md.el | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/lisp/ox-md.el b/lisp/ox-md.el
index b8c4704..a176a8f 100644
--- a/lisp/ox-md.el
+++ b/lisp/ox-md.el
@@ -181,10 +181,11 @@ channel."
   "Transcode EXAMPLE-BLOCK element into Markdown format.
 CONTENTS is nil.  INFO is a plist used as a communication
 channel."
-  (replace-regexp-in-string
-   "^" "    "
-   (org-remove-indentation
-    (org-export-format-code-default example-block info))))
+  (concat
+   "```" (org-element-property :language example-block)
+   "\n"
+   (org-export-format-code-default example-block info)
+   "```"))

 (defun org-md-export-block (export-block contents info)
   "Transcode a EXPORT-BLOCK element from Org to Markdown.
-- 
2.9.3 (Apple Git-75)