Fix brackets in links in markdown lexer (#1444)

When hiding brackets inside links (with backticks),
coloring was breaking. This makes the lexer search
for the open bracket or paren that starts the url
or reference link

lib/rouge/lexers/markdown.rb

@@ -129,7 +129,7 @@ def html
       end
 
       state :link do
-        rule %r/(\[)(#{edot}*?)(\])/ do
+        rule %r/(\[)(#{edot}*?)(?=\][\[(])/ do
           groups Punctuation, Str::Symbol, Punctuation
           pop!
         end