Fix brackets in links in Markdown lexer (#1445)

Brackets (with backticks) inside link text was not being highlighted
correctly in the Markdown lexer. This commit causes the lexer to search
for a closing bracket that is followed by either an opening bracket
(indicating this is a reference-style link) or an opening parenthesis
(indicating this is an inline link).

lib/rouge/lexers/markdown.rb

@@ -102,7 +102,7 @@ def html
         end
 
         # links and images
-        rule %r/(!?\[)(#{edot}*?)(\])/ do
+        rule %r/(!?\[)(#{edot}*?)(\])(?=[\[(])/ do
           groups Punctuation, Name::Variable, Punctuation
           push :link
         end
@@ -117,14 +117,16 @@ def html
         rule %r/<.*?@.+[.].+>/, Name::Variable
         rule %r[<(https?|mailto|ftp)://#{edot}*?>], Name::Variable
 
-
         rule %r/[^\\`\[*\n&<]+/, Text
 
         # inline html
         rule(/&\S*;/) { delegate html }
         rule(/<#{edot}*?>/) { delegate html }
         rule %r/[&<]/, Text
 
+        # An opening square bracket that is not a link
+        rule %r/\[/, Text
+
         rule %r/\n/, Text
       end
 

spec/visual/samples/markdown

@@ -1078,3 +1078,16 @@ can't highlight me
 ```` console
 $ hello "world"
 ````
+
+````markdown
+Here is an example of [feature](../feature.md):
+
+```console
+$ echo "Sample feature output"
+```
+````
+
+[This is a link `with backticks`](example.com)
+[This is a link to a TOML section `[with brackets]` (and backticks)](example.com)
+[This is a link to a TOML section `[[with double brackets]]` (and backticks)](example.com)
+[This is not a link `with backticks`] (example.com)