Support shift selection for paragraph and block movement

See GH-191.  Thanks to Brendan Rocks for reporting this.

<#191 (comment)>

markdown-mode.el

@@ -6585,7 +6585,7 @@ the surrounding context in light of Markdown syntax.  For that, see
   "Move the point to the start of the current paragraph.
 With argument ARG, do it ARG times; a negative argument ARG = -N
 means move forward N blocks."
-  (interactive "p")
+  (interactive "^p")
   (or arg (setq arg 1))
   (if (< arg 0)
       (markdown-forward-paragraph (- arg))
@@ -6639,7 +6639,7 @@ means move forward N blocks."
   "Move forward to the next end of a paragraph.
 With argument ARG, do it ARG times; a negative argument ARG = -N
 means move backward N blocks."
-  (interactive "p")
+  (interactive "^p")
   (or arg (setq arg 1))
   (if (< arg 0)
       (markdown-backward-paragraph (- arg))
@@ -6690,7 +6690,7 @@ Moves across complete code blocks, list items, and blockquotes,
 but otherwise stops at blank lines, headers, and horizontal
 rules.  With argument ARG, do it ARG times; a negative argument
 ARG = -N means move forward N blocks."
-  (interactive "p")
+  (interactive "^p")
   (or arg (setq arg 1))
   (if (< arg 0)
       (markdown-forward-block (- arg))
@@ -6737,7 +6737,7 @@ Moves across complete code blocks, list items, and blockquotes,
 but otherwise stops at blank lines, headers, and horizontal
 rules.  With argument ARG, do it ARG times; a negative argument
 ARG = -N means move backward N blocks."
-  (interactive "p")
+  (interactive "^p")
   (or arg (setq arg 1))
   (if (< arg 0)
       (markdown-backward-block (- arg))