Command line option for strict mode

Doesn't actually do anything yet. Also implemented a dummy '--strict'
command line option in text_diff.cpp, as I couldn't work out how to pass
'--strict' to quickbook and not to text_diff in the tests. So instead
just make text_diff ignore it.

src/state.cpp

@@ -46,6 +46,7 @@ namespace quickbook
         , callout_depth(0)
         , dependencies()
         , explicit_list(false)
+        , strict_mode(false)
 
         , imported(false)
         , macro()