Allow toggling of syntax concealing (#99)

Add `let g:markdown_syntax_conceal = 0` to your vimrc

README.markdown

@@ -16,6 +16,10 @@ documents you can enable it in your `.vimrc` like so:
 
     let g:markdown_fenced_languages = ['html', 'python', 'bash=sh']
 
+To disable markdown syntax concealing add the following to your vimrc:
+
+    let g:markdown_syntax_conceal = 0
+
 ## License
 
 Copyright © Tim Pope.  Distributed under the same terms as Vim itself.