Prohibit setext headers from starting with spaces or tabs

Fixes an error in the setext header regular expressions. The hyphen
needs to be at the end of the class so that it isn't interpreted as a
range.  This also adds tab as a prohibited character.

This closes GH-143 and is related to GH-139. Thanks to Howard Melman
for reporting these issues.

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
-  "^\\(?:\\([^\r\n- ].*\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
+  "^\\(?:\\([^\r\n\t -].*\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
   "Regexp identifying Markdown headings.
 Group 1 matches the text of a setext heading.
 Group 2 matches the underline of a level-1 setext heading.
@@ -1287,7 +1287,7 @@ Group 5 matches the text, without surrounding whitespace, of an atx heading.
 Group 6 matches the closing hash marks of an atx heading.")
 
 (defconst markdown-regex-header-setext
-  "^\\([^\r\n- ].*\\)\n\\(=+\\|-+\\)$"
+  "^\\([^\r\n\t -].*\\)\n\\(=+\\|-+\\)$"
   "Regular expression for generic setext-style (underline) headers.")
 
 (defconst markdown-regex-header-atx