Setext headers may not begin with hyphens

Prohibit this so that there is no ambiguity between setext headings
and in-progress lists (e.g., when the point is at X below)

- item 1
-X

Closes GH-139.

markdown-mode.el

@@ -1277,7 +1277,7 @@ Group 2 matches only the label, without the surrounding markup.
 Group 3 matches the closing square bracket.")
 
 (defconst markdown-regex-header
-  "^\\(?:\\(.+?\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
+  "^\\(?:\\([^\r\n-].*\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
   "Regexp identifying Markdown headings.
 Group 1 matches the text of a setext heading.
 Group 2 matches the underline of a level-1 setext heading.
@@ -1319,7 +1319,7 @@ Group 6 matches the closing hash marks of an atx heading.")
   "Regular expression for level 2 setext-style (underline) headers.")
 
 (defconst markdown-regex-header-setext
-  "^\\(.+\\)\n\\(\\(?:=\\|-\\)+\\)$"
+  "^\\([^\r\n-].*\\)\n\\(=+\\|-+\\)$"
   "Regular expression for generic setext-style (underline) headers.")
 
 (defconst markdown-regex-header-atx