RST reader: Allow :: before lhs code block.

The RST spec requires the :: before verbatim blocks.
This :: should not be treated as literal colons.
Resolves Issue #189.

git-svn-id: https://pandoc.googlecode.com/svn/trunk@1668 788f1e2b-df1e-0410-8736-df70ead52e1b

src/Text/Pandoc/Readers/RST.hs

@@ -345,6 +345,7 @@ customCodeBlock = try $ do
 lhsCodeBlock :: GenParser Char ParserState Block
 lhsCodeBlock = try $ do
   failUnlessLHS
+  optional codeBlockStart
   pos <- getPosition
   when (sourceColumn pos /= 1) $ fail "Not in first column"
   lns <- many1 birdTrackLine